H transport initialisation refactoring

[jhdark]

Proposed changes

We habe a minor python bug when multiple instances of the same class are within the same script, and with attributes that default to empty lists, if a value is appended to this list, it becomes the default for subsequent instances like so:

model_1 = F.HydrogenTransportProblem()
model_1.reactions.append(1)

model_2 = F.HydrogenTransportProblem()
print(model_2.reactions)

gives:

[1]

The changes presented fixes this by altering the attribute defaults in HydrogenTransportProblem from

class HydrogenTransportProblem:
    def __init__(self, attribute=[]):
        self.attribute = attribute

to:

class HydrogenTransportProblem:
    def __init__(self, attribute=None):
        self.attribute = attribute

        if self.attribute is None:
            self.attribute = []

and this seems to fix it. Found this in a handy pitfall guide: https://docs.python-guide.org/writing/gotchas/

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)

Further comments

If this is a relatively large or complex change, kick off the discussion by explaining why you chose the solution you did and what alternatives you considered, etc...

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (71039f3) 98.87% compared to head (93c92c1) 98.87%.

Additional details and impacted files

@@           Coverage Diff            @@
##           fenicsx     #661   +/-   ##
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  Coverage    98.87%   98.87%           
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  Files           23       23           
  Lines          974      974           
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  Hits           963      963           
  Misses          11       11           

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[RemDelaporteMathurin]

This looks great to me, thanks for fixing this. It is a common pitfall in python!

Just have a few comments for future proofing 🚀

[RemDelaporteMathurin]

Nice hack, I didn't know of this way of doing things.
Does it work for all python versions?

Also, we may want to add a comment to say what this does for people like me who don't know this