Refactor redundant legacy

[samaloney]

Remove redundant 'legacy' in module from ~sunkit_spex.legacy.fitting_legacy to sunkit_spex.legacy.fitting.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 38.88889% with 66 lines in your changes missing coverage. Please review.

Please upload report for BASE (main@3a9a4a4). Learn more about missing BASE report.

Files	Patch %	Lines
sunkit_spex/legacy/fitting/data_loader.py	10.34%	52 Missing :warning:
sunkit_spex/legacy/fitting/instruments.py	38.09%	13 Missing :warning:
sunkit_spex/legacy/fitting/fitter.py	90.00%	1 Missing :warning:

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #158   +/-   ##
=======================================
  Coverage        ?   60.50%           
=======================================
  Files           ?       25           
  Lines           ?     3408           
  Branches        ?        0           
=======================================
  Hits            ?     2062           
  Misses          ?     1346           
  Partials        ?        0           

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[samaloney]

Note I haven't update the notebooks as they will be removed in #153

[samaloney]

Reviews please

[nabobalis]

Why was it called legacy originally?

[samaloney]

Needed a way to delineate older code which won't be updated and will eventually be removed from what we are now trying to do