created shared Problem class for HeatTransferProblem and HTransportProblem

[rekomodo]

Solve #745

Proposed changes

Created a common Problem class to house functionality common to both HeatTransferProblem and HTransportProblem.

Types of changes

What types of changes does your code introduce to FESTIM?

• [ ] Bugfix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [x] Code refactoring
• [ ] Documentation Update (if none of the other choices apply)
• [ ] New tests

Checklist

• [x] Black formatted
• [x] Unit tests pass locally with my changes
• [ ] I have added tests that prove my fix is effective or that my feature works
• [ ] I have added necessary documentation (if appropriate)

Further comments

Problem class has no docstring yet.

[codecov[bot]]

Codecov Report

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

Project coverage is 98.92%. Comparing base (4e2afe8) to head (b29e8d9). Report is 211 commits behind head on fenicsx.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## fenicsx #857 +/- ## =========================================== - Coverage 98.95% 98.92% -0.04% =========================================== Files 35 36 +1 Lines 1631 1580 -51 =========================================== - Hits 1614 1563 -51 Misses 17 17 ```

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