✨ feat(dynamics): class-based relaxation time calculators

[nstarman]

This PR changes from the verbose

class MethodFlag: ...

def relax_time_specific_method(): ...

@dispatch
def relaxation_time(type[MethodFlag], ...):
    return relax_time_specific_method()

to the simpler

class Method: ...
    def __call__(*args): ...


@dispatch
def relaxation_time(type[MethodFlag], ...):
    return Method(params)(*args)

What do you think?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.85%. Comparing base (9ff983d) to head (4a486b2).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #749      +/-   ##
==========================================
+ Coverage   95.84%   95.85%   +0.01%     
==========================================
  Files         158      158              
  Lines        5963     5989      +26     
==========================================
+ Hits         5715     5741      +26     
  Misses        248      248              

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