Replace documentation references to SimpleModel with FunctionModel

[omsai]

SimpleModel was removed in 3fc1a52c5d124f62add2cf7b245fb582ce2d6cea

• I confirmed that the sphinx docs build as expected.
• FYI the pre-commit hook recommended by CONTRIBUTE.rst fails as follows:

  
  [INFO] Initializing environment for https://github.com/psf/black.
  [INFO] Initializing environment for https://github.com/pycqa/isort.
  [INFO] Initializing environment for https://github.com/nbQA-dev/nbQA.
  [INFO] Initializing environment for https://github.com/nbQA-dev/nbQA:black.
  [INFO] Initializing environment for https://github.com/nbQA-dev/nbQA:pyupgrade.
  [INFO] Initializing environment for https://github.com/nbQA-dev/nbQA:isort.
  [INFO] Initializing environment for https://github.com/pre-commit/pre-commit-hooks.
  [INFO] Installing environment for https://github.com/psf/black.
  [INFO] Once installed this environment will be reused.
  [INFO] This may take a few minutes...
  [INFO] Installing environment for https://github.com/pycqa/isort.
  [INFO] Once installed this environment will be reused.
  [INFO] This may take a few minutes...
  [INFO] Installing environment for https://github.com/nbQA-dev/nbQA.
  [INFO] Once installed this environment will be reused.
  [INFO] This may take a few minutes...
  [INFO] Installing environment for https://github.com/nbQA-dev/nbQA.
  [INFO] Once installed this environment will be reused.
  [INFO] This may take a few minutes...
  [INFO] Installing environment for https://github.com/nbQA-dev/nbQA.
  [INFO] Once installed this environment will be reused.
  [INFO] This may take a few minutes...
  [INFO] Installing environment for https://github.com/pre-commit/pre-commit-hooks.
  [INFO] Once installed this environment will be reused.
  [INFO] This may take a few minutes...
  black....................................................................Passed
  isort (python)...........................................................Passed
  isort (cython).......................................(no files to check)Skipped
  isort (pyi)..........................................(no files to check)Skipped
  nbqa-black...........................................(no files to check)Skipped
  nbqa-pyupgrade.......................................(no files to check)Skipped
  nbqa-isort...........................................(no files to check)Skipped
  check yaml...........................................(no files to check)Skipped
  check for added large files..............................................Passed
  check for merge conflicts................................................Passed
  check for broken symlinks............................(no files to check)Skipped
  trim trailing whitespace.................................................Passed
  fix end of files.........................................................Passed
  detect private key.......................................................Passed
  Check style..............................................................Failed
  - hook id: style
  - exit code: 1

flake8: install_deps> python -I -m pip install 'black>=22.3.0' 'flake8-bandit>=4.1.1' 'flake8-black>=0.2.3' 'flake8-bugbear>=22.8.23' 'flake8-colors>=0.1.6' 'flake8-comprehensions>=3.2.3' 'flake8-isort>=4.0.0' 'flake8-print>=5.0.0' 'flake8>=3.8.3' flake8: commands[0]> flake8 pyabc test test_performance setup.py Unable to find qualified name for module: setup.py pyabc/acceptor/init.py:8:1: BLK100 Black would make changes. pyabc/acceptor/acceptor.py:443:27: BLK100 Black would make changes. pyabc/distance/distance.py:72:13: BLK100 Black would make changes. pyabc/distance/kernel.py:2:1: BLK100 Black would make changes. pyabc/distance/scale.py:34:1: BLK100 Black would make changes. pyabc/predictor/predictor.py:484:13: BLK100 Black would make changes. pyabc/storage/migrations/versions/1_20210219_add_particles_proposal_id.py:8:1: BLK100 Black would make changes. pyabc/util/par_trafo.py:97:13: BLK100 Black would make changes. test/base/test_samplers.py:231:69: BLK100 Black would make changes. test_performance/test_samplerperf.py:163:69: BLK100 Black would make changes. flake8: exit 1 (3.28 seconds) /Users/pnanda/src/pyABC> flake8 pyabc test test_performance setup.py pid=71418 flake8: FAIL code 1 (6.92=setup[3.64]+cmd[3.28] seconds) evaluation failed :( (6.96 seconds)

- The above Black-related issues don't flag the `pyabc/model/model.py` that I changed, so I uninstalled the commit hook to create the PR.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 81.65%. Comparing base (6446a77) to head (0d1305b). Report is 235 commits behind head on develop.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #629 +/- ## =========================================== - Coverage 84.08% 81.65% -2.44% =========================================== Files 91 141 +50 Lines 5260 8535 +3275 =========================================== + Hits 4423 6969 +2546 - Misses 837 1566 +729 ```

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[stephanmg]

Thanks for the fix.