Temperature can be given as float or sp.Expr

[RemDelaporteMathurin]

Proposed changes

This fixes #741 by adding a setter/getter to the T attribute of Simulation.

Now, users can simply give the temperature as a numerical value or sympy Expression without having to create a Temperature object.

Little QoL improvement!

Types of changes

What types of changes does your code introduce to FESTIM?

• [ ] Bugfix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] 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
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] I have added necessary documentation (if appropriate)

[codecov[bot]]

Codecov Report

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

Project coverage is 99.52%. Comparing base (fb2c99f) to head (6cb7559). Report is 196 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #770 +/- ## ======================================= Coverage 99.51% 99.52% ======================================= Files 58 58 Lines 2488 2501 +13 ======================================= + Hits 2476 2489 +13 Misses 12 12 ```

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