Implement mulitprocessing support for GPy and sklearn

[kjappelbaum]

• implements multiprocessing support for GPy and sklearn (models for different objectives can be trained in parallel by setting n_jobs in the class initialization)
• added note in docs about the design pattern for implementing concurrent operations
• for coregionalized models this does not make sense

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation

Actions (for code changes)

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

[kjappelbaum]

Codecov Report

Merging #41 into master will increase coverage by 0.15%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #41      +/-   ##
==========================================
+ Coverage   96.68%   96.84%   +0.15%     
==========================================
  Files          10       10              
  Lines         574      603      +29     
==========================================
+ Hits          555      584      +29     
  Misses         19       19              

Flag	Coverage Δ
#unittests	96.84% <100.00%> (+0.15%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
pypal/pal/pal_gpy.py	100.00% <100.00%> (ø)
pypal/pal/pal_sklearn.py	100.00% <100.00%> (ø)
pypal/pal/validate_inputs.py	93.25% <100.00%> (+0.40%)	⬆️

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