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 :white_check_mark:

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

:exclamation: 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

```diff @@ 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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