Adaptive stepsize

[RemDelaporteMathurin]

Proposed changes

This is an attempt at adding adaptive timestepping.

I've taken a similar approach to MOOSE where the users give a target number of iterations and a growth factor and a cutback factor.

If the number of newton iterations is larger than the target number, the stepsize is reduced (multiplied by cutback factor). Inversely when it's lower than the target, the stepsize is increased (growth factor).

@jhdark I've noticed we have the concept of the stepsize living in different places.
First we have HTransportProblem.settings.stepsize AND HTransportProblem.dt (which is just the fenicsx.Constant object).
I feel like we should remove this redundancy to improve clarity. Doesn't have to be in this PR but I felt like we could kick off the discussion now

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 ✅

Comparison is base (d601d06) 98.87% compared to head (a816839) 98.90%.

Additional details and impacted files

@@             Coverage Diff             @@
##           fenicsx     #654      +/-   ##
===========================================
+ Coverage    98.87%   98.90%   +0.03%     
===========================================
  Files           23       23              
  Lines          974     1009      +35     
===========================================
+ Hits           963      998      +35     
  Misses          11       11              

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[jhdark]

Think we can change a few small things here, but otherwise, its looking good and will be incredibly useful

[jhdark]

Looks good