WIP: enable multiple hyperparameter groups and improve mp performance

[nw13slx]

• this branch enables multi hyperparameters and allows partial optimization
  • with a new mc_sephyps kernel
  • multihyps flag and hyps_mask members in GP class
• reduce memory consumption and improve mp performance by
  • reduce data passing through mp while computing the covariance matrix
  • avoid nesting of threads

[codecov-io]

Codecov Report

Merging #139 into master will decrease coverage by 2.29%.
The diff coverage is 46.34%.

@@            Coverage Diff            @@
##           master     #139     +/-   ##
=========================================
- Coverage   58.36%   56.06%   -2.3%     
=========================================
  Files          31       35      +4     
  Lines        4813     5797    +984     
=========================================
+ Hits         2809     3250    +441     
- Misses       2004     2547    +543

Impacted Files	Coverage Δ
flare/kernels/kernels.py	21.62% <ø> (ø)
flare/mc_simple.py	100% <100%> (+78.28%)	⬆️
flare/kernels/__init__.py	100% <100%> (ø)
flare/kernels/mc_sephyps.py	21.49% <21.49%> (ø)
flare/kernels/mc_simple.py	21.71% <21.71%> (ø)
flare/dft_interface/vasp_util.py	95.34% <33.33%> (ø)	⬆️
flare/mgp/utils.py	21.53% <33.33%> (-0.38%)	⬇️
flare/dft_interface/__init__.py	75% <50%> (ø)	⬆️
flare/kernels/utils.py	58.82% <58.82%> (ø)
flare/gp.py	79.8% <72.15%> (-9.65%)	⬇️
... and 11 more

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