Parametrize struct definitions of random matrix ensemble types

[apkille]

See #91.

[codecov]

Codecov Report

Attention: Patch coverage is 90.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 42.65%. Comparing base (37f177b) to head (6f55049).

Files with missing lines	Patch %	Lines
src/GaussianEnsembles.jl	88.00%	3 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #96   +/-   ##
=======================================
  Coverage   42.65%   42.65%           
=======================================
  Files          12       12           
  Lines         830      830           
=======================================
  Hits          354      354           
  Misses        476      476           

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