Raise error if first element returns inf in _minimize_local_discrete

[veni-vidi-vici-dormivi]

This is to avoid if the first element returns inf, the last element is returned.

Note that for the application in find_localized_empirical_covariance() the whole thing implicitly assumes that the localization radii are ordered and increasing. And that once a localization radius leads to a singular covariance matrix all higher ones do too (reasonably).

• Closes Bug in _minimize_local_discrete #616
• Tests added
• Fully documented, including CHANGELOG.rst

[veni-vidi-vici-dormivi]

Note also that we do not use minimize_local_discrete anywhere else at the moment.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.44%. Comparing base (681fa61) to head (fa715e8).
Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #617      +/-   ##
==========================================
+ Coverage   80.40%   80.44%   +0.03%     
==========================================
  Files          49       49              
  Lines        3082     3078       -4     
==========================================
- Hits         2478     2476       -2     
+ Misses        604      602       -2     

Flag	Coverage Δ
unittests	80.44% <100.00%> (+0.03%)	⬆️

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[veni-vidi-vici-dormivi]

Closing again in favor of #493

[veni-vidi-vici-dormivi]

Okay leaving this here in case we decide to not merge #493