Error message when temperature is not defined

[RemDelaporteMathurin]

Proposed changes

I've noticed that it's easy to miss the temperature field when setting up a model. And the error message that's produced isn't straightforward:

Traceback (most recent call last):
  File "/home/remidm/FESTIM/mwe_2.py", line 19, in <module>
    my_model.initialise()
  File "/home/remidm/FESTIM/festim/generic_simulation.py", line 264, in initialise
    self.attribute_source_terms()
  File "/home/remidm/FESTIM/festim/generic_simulation.py", line 173, in attribute_source_terms
    self.T.sources = []
    ^^^^^^^^^^^^^^
AttributeError: 'NoneType' object has no attribute 'sources'

Instead, this PR adds a clearer error message upon calling initialise()

Traceback (most recent call last):
  File "/home/remidm/FESTIM/mwe_2.py", line 18, in <module>
    my_model.initialise()
  File "/home/remidm/FESTIM/festim/generic_simulation.py", line 257, in initialise
    raise AttributeError("Temperature is not defined")
AttributeError: Temperature is not defined

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 (24420e6) to head (d4a8d6e). Report is 6 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #768 +/- ## ========================================== + Coverage 99.51% 99.52% +0.01% ========================================== Files 58 58 Lines 2468 2523 +55 ========================================== + Hits 2456 2511 +55 Misses 12 12 ```

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