Add methods for calling multi-response gaussian family

[BenjaminDoran]

Adds methods to call glmnet(X, Y, MvNormal()) for multi-response normal problems.

I added tests to make sure the methods are producing the same answers as the R version.

Should close my issue #65

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 62.31884% with 26 lines in your changes are missing coverage. Please review.

Project coverage is 88.97%. Comparing base (47da7a0) to head (582ed9b).

Files	Patch %	Lines
src/MultiResponseNet.jl	58.06%	26 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #66      +/-   ##
==========================================
- Coverage   93.61%   88.97%   -4.65%     
==========================================
  Files           3        4       +1     
  Lines         470      535      +65     
==========================================
+ Hits          440      476      +36     
- Misses         30       59      +29     

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[JackDunnNZ]

Closing and opening to retrigger actions