Simulation time milestones

[RemDelaporteMathurin]

Proposed changes

Often it is required to have the simulation hit some time milestones.
For instance, I want the simulation to simulate t=100s explicitely, not 80s, not 150s, 100s.

So far, we could use TXT exports that would modify the stepsize accordingly.
We can now do it directly in festim.Stepsize by specifying milestones.

Usage

my_model.dt = F.Stepsize(
    initial_value=15,
    milestones=[90, 100],
)

In the current implementation, the TXT export don't modify the stepsize anymore. Meaning that users must add milestones if they want to be sure to export the TXT at these specific times.

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 lines are covered by tests ✅

Comparison is base (616d96c) 99.07% compared to head (9035525) 98.90%.
Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #568      +/-   ##
==========================================
- Coverage   99.07%   98.90%   -0.18%     
==========================================
  Files          56       56              
  Lines        2053     2096      +43     
==========================================
+ Hits         2034     2073      +39     
- Misses         19       23       +4     

Files	Coverage Δ
festim/exports/exports.py	97.72% <100.00%> (+0.05%)	⬆️
festim/exports/txt_export.py	98.03% <100.00%> (-1.97%)	⬇️
festim/generic_simulation.py	100.00% <100.00%> (ø)
festim/h_transport_problem.py	93.25% <100.00%> (ø)
festim/stepsize.py	100.00% <100.00%> (ø)

... and 6 files with indirect coverage changes

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[jhdark]

This looks great to me, I know some people who've asked for this feature.