Flexible error handling of empty comparers

[ecomodeller]

This PR introduces flexible error handling for cases where observations and model results do not overlap. It adds a new parameter, obs_no_overlap, to the match function and related methods, allowing users to specify whether to ignore, raise an error, or issue a warning when no overlap is found.

This makes it easier for the end-user but the current implementation adds complexity by returning None from several methods. A more elegant solution would be to use the Null object pattern.