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)
• 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

• Black formatted
• 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)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.52%. Comparing base (24420e6) to head (d4a8d6e).
Report is 233 commits behind head on main.

Additional details and impacted files

@@            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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[jhdark]

Looks good to me, we could make a list of other common warnings that could be added as a good first issue too.