Iterate method

[RemDelaporteMathurin]

As discussed in #620 , this adds an .iterate method to HydrogenTransportProblem to refactor the run method a bit.

Temporarily, I added a skip_post_processing boolean flag to the method (defaulting to False) to simplify the test since most of what we have in the post-processing atm will go away.

I also added a test for iterate. It doesn't test everything but tried to keep it small.

Notes:

• we would need to add unit tests for the methods of HydrogenTransportProblem
• I added a bunch of new undocumented attributes for communication between run and iterate. These will be removed as soon as we have a proper post-processing step

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (b975a64) 98.17% compared to head (a9fa55b) 98.18%.

Additional details and impacted files

@@           Coverage Diff            @@
##           fenicsx     #621   +/-   ##
========================================
  Coverage    98.17%   98.18%           
========================================
  Files           16       16           
  Lines          549      552    +3     
========================================
+ Hits           539      542    +3     
  Misses          10       10           

Files	Coverage Δ
festim/hydrogen_transport_problem.py	99.48% <100.00%> (+<0.01%)	⬆️

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[jhdark]

Looks good to me