feat: add basic GPflow support

[kjappelbaum]

This GPFlow support is really basic as the range of types of models you can build with GPFlow is just too wide to handle in one class.

I somehow could not train the GPRs in separate processes, using ThreadPoolExecutor for this reason, but this is more of a temporary fix.

• Breaking change (fix or feature that would cause existing functionality to change)

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Documentation
• Developer tools
• Refactoring
• Test

Actions (for code changes)

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

[codecov-commenter]

Codecov Report

Merging #178 (dbc5b12) into master (0ae381c) will increase coverage by 0.15%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #178      +/-   ##
==========================================
+ Coverage   96.07%   96.22%   +0.15%     
==========================================
  Files          15       16       +1     
  Lines        1120     1167      +47     
==========================================
+ Hits         1076     1123      +47     
  Misses         44       44              

Flag	Coverage Δ
unittests	96.22% <100.00%> (+0.15%)	⬆️

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Impacted Files	Coverage Δ
pyepal/pal/pal_gpflowgpr.py	100.00% <100.00%> (ø)

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[byooooo]

lgtm