[WIP] Interpolation overhaul

[alanlujan91]

Please ensure your pull request adheres to the following guidelines:

• Tests for new functionality/models or Tests to reproduce the bug-fix in code.
• Updated documentation of features that add new functionality.
• Update CHANGELOG.md with major/minor changes.

[sbenthall]

#1242 is about the EconForge interpolator as an additional backend to multivariate interpolation?

[llorracc]

Small comment: My pref is to be more systematic about naming, and have the general followed by the specific. Like, it should be interp_bilinear, interp_decay, interp_linear. Also, is LinearFast an interpolator? If so it should be interp_linear_fast

[codecov]

Codecov Report

Patch coverage: 53.90% and project coverage change: -0.54 ⚠️

Comparison is base (2209109) 72.55% compared to head (18f4329) 72.01%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1203      +/-   ##
==========================================
- Coverage   72.55%   72.01%   -0.54%     
==========================================
  Files          78       85       +7     
  Lines       13009    13391     +382     
==========================================
+ Hits         9439     9644     +205     
- Misses       3570     3747     +177     

Impacted Files	Coverage Δ
HARK/interpolation/_econforge.py	90.36% <ø> (ø)
HARK/interpolation/_hark.py	43.31% <ø> (ø)
HARK/interpolation/tests/test_hark.py	100.00% <ø> (ø)
HARK/interpolation/_sklearn.py	39.82% <39.82%> (ø)
HARK/interpolation/_multi.py	41.98% <41.98%> (ø)
HARK/interpolation/_scipy.py	85.18% <85.18%> (ø)
HARK/interpolation/__init__.py	100.00% <100.00%> (ø)
HARK/interpolation/tests/test_econforge.py	100.00% <100.00%> (ø)
HARK/interpolation/tests/test_multi.py	100.00% <100.00%> (ø)
HARK/interpolation/tests/test_scipy.py	100.00% <100.00%> (ø)
... and 1 more

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