Adding metric wrapper classes

[drewoldag]

Problem & Solution Description (including issue #)

The variety of different metric APIs that are present in qp currently make it challenging to write consistent evaluator stages in RAIL. By introducing wrapper classes we can abstract and unify the API (as much as possible) while making it easier to interface with RAIL evaluator stages.

TODO

Need to add simple unit test coverage for the classes
Include PIT Metric wrapper class.

Code Quality

• My code follows the code style of this project
• I have written unit tests or justified all instances of #pragma: no cover; in the case of a bugfix, a new test that breaks as a result of the bug has been added
• My code contains relevant comments and necessary documentation for future maintainers; the change is reflected in applicable demos/tutorials (with output cleared!) and added/updated docstrings use the NumPy docstring format
• Any breaking changes, described above, are accompanied by backwards compatibility and deprecation warnings

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (a578c66) 99.95% compared to head (838ba13) 99.95%.

Additional details and impacted files

@@           Coverage Diff            @@
##             main     #203    +/-   ##
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  Coverage   99.95%   99.95%            
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  Files          33       35     +2     
  Lines        2294     2463   +169     
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+ Hits         2293     2462   +169     
  Misses          1        1            

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[eacharles]

Overall looks good, I had a couple minor comments about adding docstrings to the base classes and if the placeholder functions should be no-ops (i.e., just pass) instead of NotImplemented. Up to you how if you want to implement those changes.