Improve input checking: prefer checkmate for custom checks

[jamesmbaazam]

This PR closes #147 and fixes #162. It implements a few changes to improve how input checking is done:

• Removes custom checks check_offspring_valid() and check_ntrees_valid() in favour of checkmate functions.
• Refactors check_generation_time_valid() and check_offspring_func() valid to use checkmate::assert_* instead of !checkmate::test_*.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (d499296) 99.11% compared to head (1eabf96) 99.10%.
Report is 2 commits behind head on main.

❗ Current head 1eabf96 differs from pull request most recent head 5bf2e13. Consider uploading reports for the commit 5bf2e13 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #174      +/-   ##
==========================================
- Coverage   99.11%   99.10%   -0.01%     
==========================================
  Files           8        8              
  Lines         562      556       -6     
==========================================
- Hits          557      551       -6     
  Misses          5        5              

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