Add BasicGITT model

[brosaplanella]

Description

Add BasicGITT model so we can do some progress with GITT while I figure out how to implement it in PyBaMM.

Type of change

Please add a line in the relevant section of CHANGELOG.md to document the change (include PR #) - note reverse order of PR #s. If necessary, also add to the list of breaking changes.

• [x] New feature (non-breaking change which adds functionality)
• [ ] Optimization (back-end change that speeds up the code)
• [ ] Bug fix (non-breaking change which fixes an issue)

Key checklist:

• [x] No style issues: $ flake8
• [x] All tests pass: $ python run-tests.py --unit
• [x] The documentation builds: $ cd docs and then $ make clean; make html

You can run all three at once, using $ python run-tests.py --quick.

Further checks:

• [x] Code is commented, particularly in hard-to-understand areas
• [x] Tests added that prove fix is effective or that feature works

[codecov[bot]]

Codecov Report

Patch coverage: 95.52% and project coverage change: +4.36% :tada:

Comparison is base (425303d) 83.63% compared to head (04ecc98) 88.00%. Report is 34 commits behind head on develop.

:exclamation: Current head 04ecc98 differs from pull request most recent head df7f404. Consider uploading reports for the commit df7f404 to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #50 +/- ## =========================================== + Coverage 83.63% 88.00% +4.36% =========================================== Files 13 15 +2 Lines 336 400 +64 =========================================== + Hits 281 352 +71 + Misses 55 48 -7 ``` | [Files Changed](https://app.codecov.io/gh/paramm-team/pybamm-param/pull/50?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=paramm-team) | Coverage Δ | | |---|---|---| | [pbparam/optimisation\_problems/gitt.py](https://app.codecov.io/gh/paramm-team/pybamm-param/pull/50?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=paramm-team#diff-cGJwYXJhbS9vcHRpbWlzYXRpb25fcHJvYmxlbXMvZ2l0dC5weQ==) | `21.21% <0.00%> (+21.21%)` | :arrow_up: | | [pbparam/\_\_init\_\_.py](https://app.codecov.io/gh/paramm-team/pybamm-param/pull/50?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=paramm-team#diff-cGJwYXJhbS9fX2luaXRfXy5weQ==) | `100.00% <100.00%> (ø)` | | | [pbparam/models/basic\_gitt.py](https://app.codecov.io/gh/paramm-team/pybamm-param/pull/50?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=paramm-team#diff-cGJwYXJhbS9tb2RlbHMvYmFzaWNfZ2l0dC5weQ==) | `100.00% <100.00%> (ø)` | | | [pbparam/models/weppner\_huggins.py](https://app.codecov.io/gh/paramm-team/pybamm-param/pull/50?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=paramm-team#diff-cGJwYXJhbS9tb2RlbHMvd2VwcG5lcl9odWdnaW5zLnB5) | `100.00% <100.00%> (ø)` | |

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