MBM: optional GP-based M sampling on InfiniteGDP models

[dnguyen227]

Implementation of MBM

using AbstractGPs 

optimize!(model, gdp_method = MBM(HiGHS.Optimizer)) 

MBM(HiGHS.Optimizer, M_sampler = :exact)                        # every support, as before
MBM(HiGHS.Optimizer, M_sampler = GPSampler(budget = 0.1))       # solve 10% of supports
MBM(HiGHS.Optimizer, M_sampler = GPSampler(kappa = 4.0))        # more conservative M

Not sure how to work around needing to make a new extension.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.53%. Comparing base (a4ebe8a) to head (fa4a7ca).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #139      +/-   ##
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+ Coverage   99.51%   99.53%   +0.01%     
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  Files          17       18       +1     
  Lines        2061     2143      +82     
==========================================
+ Hits         2051     2133      +82     
  Misses         10       10              

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