[CUSOLVER] Interface gesv! and gels!

[amontoison]

• Interface gesv and gels for solving dense linear problems AX = B (square and rectangular).
  These routines are quite complex because they can use low precision to perform some operations and provide many options to the user.
• I also created a file helpers.jl to store all structures for CUSOLVER in one file. We already did that in lib/cusparse.

[codecov]

Codecov Report

Attention: Patch coverage is 74.46809% with 48 lines in your changes missing coverage. Please review.

Project coverage is 73.65%. Comparing base (19a08ef) to head (b93fef3).

Files with missing lines	Patch %	Lines
lib/cusolver/base.jl	58.33%	20 Missing ⚠️
lib/cusolver/helpers.jl	69.69%	20 Missing ⚠️
lib/cusolver/dense.jl	89.18%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2406      +/-   ##
==========================================
- Coverage   73.69%   73.65%   -0.04%     
==========================================
  Files         156      157       +1     
  Lines       15048    15204     +156     
==========================================
+ Hits        11090    11199     +109     
- Misses       3958     4005      +47     

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