support for warmstart in HiGHS

[fneum]

It adds support for reading in solution or basis files for a warm start and functionality to write solution and basis files from solved HiGHS models.

I am currently identifying the kind of warm start (solution or basis) by the file suffix. Is this the best way? The documentation of the Model.solve() function currently states it's only a basis file that is used for the warm start (and that's how we have done it for the other solvers.)

The writing functions do not throw errors for unsolved models, so I am not handling these.

The 0 for writing the solution indicates the format of the .sol file. I am choosing raw.

[codecov]

Codecov Report

Attention: Patch coverage is 72.72727% with 3 lines in your changes missing coverage. Please review.

Project coverage is 89.66%. Comparing base (2fc1c1b) to head (169a7d9).

Files	Patch %	Lines
linopy/solvers.py	72.72%	1 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #309      +/-   ##
==========================================
- Coverage   89.72%   89.66%   -0.06%     
==========================================
  Files          16       16              
  Lines        4037     4046       +9     
  Branches      942      945       +3     
==========================================
+ Hits         3622     3628       +6     
- Misses        282      283       +1     
- Partials      133      135       +2     

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