Fix MOSEK parameter setting

[aszekMosek]

This is inspired by https://stackoverflow.com/questions/59040539/parameter-to-create-mosek-feasibility-report-with-cvxr. It fixes and simplifies MOSEK parameter settings through CVXR.

The change in line 36 fixes an actual bug which made it impossible to set MOSEK parameters at all.

The rewrite of _handle_mosek_params adapts it better to the argument types one can expect from R, removing the unnecessary parts originating from CVXPY but impossible to reach from R, and accounting for the fact that integers are doubles unless explicitly told not to. Examples are given in the comment to that method.

[codecov-io]

Codecov Report

Merging #60 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #60   +/-   ##
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  Coverage   67.37%   67.37%           
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  Files          40       40           
  Lines        4705     4705           
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  Hits         3170     3170           
  Misses       1535     1535

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[bnaras]

Thanks. This PR looks correct to me and I will merge it as soon as I can. However, just fyi, the version 1.0 branch does away with this (stop-gap) approach and uses native R packages.