Fix rand(d::GaussianLaguerre, n::Int), improve performance, add tests

[apkille]

Note that I replaced the method rand(d::GaussianLaguerre, n::dims) with rand(d::GaussianLaguerre, n::Int) to match similar rand methods for other random matrix ensembles in the package. Ping @dlfivefifty for review.

[apkille]

Fixes #87

[codecov]

Codecov Report

Attention: Patch coverage is 95.45455% with 1 line in your changes missing coverage. Please review.

Project coverage is 42.65%. Comparing base (fc98f3b) to head (29255e0).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/GaussianEnsembles.jl	95.45%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #93      +/-   ##
==========================================
+ Coverage   39.80%   42.65%   +2.84%     
==========================================
  Files          12       12              
  Lines         819      830      +11     
==========================================
+ Hits          326      354      +28     
+ Misses        493      476      -17     

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