Free mps

[mtanneau]

Adds support for parsing Free MPS format. Free MPS should cover almost all cases; the only limitation is that names should not contain any spaces.

A few points and questions:

• I expanded existing tests to run with both free and fixed format. Should we have more?
• Markers in the COLUMNS section (e.g., integrality markers) are ignored. An @error message is displayed though
• Currently, there is no automatic detection of whether the file is in fixed or free format. The format must be specified as a keyword argument to readqps (default value is :fixed).

[codecov]

Codecov Report

Merging #21 into master will decrease coverage by 0.01%.
The diff coverage is 75%.

@@            Coverage Diff             @@
##           master      #21      +/-   ##
==========================================
- Coverage    77.5%   77.48%   -0.02%     
==========================================
  Files           2        2              
  Lines         369      422      +53     
==========================================
+ Hits          286      327      +41     
- Misses         83       95      +12

Impacted Files	Coverage Δ
src/readqps.jl	77.43% <75%> (-0.02%)	⬇️

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[dpo]

I'm having trouble finding the time to review this in detail because the code doesn't look like my original code anymore... I'm trusting you. Many thanks for all those improvements!