Patch derived quantities not exported at final timestep when stepsize is small

[RemDelaporteMathurin]

Proposed changes

This patch fixes #566 by modifying the way we check if the current timestep is the final time.

Thanks @nicolasialovega for helping finding the bug.

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

Patch coverage: 100.00% and no project coverage change.

Comparison is base (6716ec5) 98.84% compared to head (abe3b09) 98.84%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #567   +/-   ##
=======================================
  Coverage   98.84%   98.84%           
=======================================
  Files          56       56           
  Lines        2072     2072           
=======================================
  Hits         2048     2048           
  Misses         24       24           

Files Changed	Coverage Δ
...m/exports/derived_quantities/derived_quantities.py	99.00% <100.00%> (ø)

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[jhdark]

This is a great addition, thanks again!