[Closes #129] Fixes issue with deepcopy in composite model

[teubert]

prog_server requires copying a model. This functionality is broken for composite model.

Fixed issue by moving more logic into setstate, which is called at construction and deepcopy (unlike init). Also added tests to catch this in the future

[github-actions[bot]]

Thank you for opening this PR. Each PR into dev requires a code review. For the code review, look at the following:

• [ ] Reviewer (someone other than author) should look for bugs, efficiency, readability, testing, and coverage in examples (if relevant).
• [ ] Ensure that each PR adding a new feature should include a test verifying that feature.
• [ ] All errors from static analysis must be resolved.
• [ ] Review the test coverage reports (if there is a change) - will be added as comment on PR if there is a change
• [ ] Review the software benchmarking results (if there is a change) - will be added as comment on PR
• [ ] Any added dependencies are included in requirements.txt, setup.py, and dev_guide.rst (this document)
• [ ] All warnings from static analysis must be reviewed and resolved - if deemed appropriate.

[codecov-commenter]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (28dd429) 83.36% compared to head (51ce763) 83.41%.

Files	Patch %	Lines
src/progpy/composite_model.py	96.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## dev #130 +/- ## ========================================== + Coverage 83.36% 83.41% +0.05% ========================================== Files 100 100 Lines 10428 10462 +34 ========================================== + Hits 8693 8727 +34 Misses 1735 1735 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.