Initialise Stepsize.value in Simulation.initialise()

[RemDelaporteMathurin]

Proposed changes

Fixes #485 by initialising re-initialising the stepsize value when calling Simulation.initialise

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

Merging #486 (dbb3707) into dev (035bcd5) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##              dev     #486   +/-   ##
=======================================
  Coverage   98.88%   98.89%           
=======================================
  Files          53       53           
  Lines        1976     1982    +6     
=======================================
+ Hits         1954     1960    +6     
  Misses         22       22           

Impacted Files	Coverage Δ
FESTIM/generic_simulation.py	100.00% <100.00%> (ø)
FESTIM/stepsize.py	100.00% <100.00%> (ø)

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[jhdark]

good stuff