Add a spline interpolator for 2d arrays

[mraspaud]

This PR adds a spline interpolator based on scipy's RectBivariate spline, which is much faster than the existing grid interpolator based on the RegularGrid interpolator, but cannot extrapolate.

• Tests added
• Tests passed
• Passes git diff origin/main **/*py | flake8 --diff
• Fully documented

[codecov]

Codecov Report

Attention: Patch coverage is 99.03846% with 1 line in your changes missing coverage. Please review.

Project coverage is 88.43%. Comparing base (f151c94) to head (ffc1628).
Report is 21 commits behind head on main.

Files with missing lines	Patch %	Lines
geotiepoints/interpolator.py	97.61%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #85      +/-   ##
==========================================
+ Coverage   87.94%   88.43%   +0.48%     
==========================================
  Files          20       20              
  Lines        1452     1522      +70     
==========================================
+ Hits         1277     1346      +69     
- Misses        175      176       +1     

Flag	Coverage Δ
unittests	88.43% <99.03%> (+0.48%)	⬆️

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[coveralls]

coverage: 87.725% (+0.5%) from 87.233%
when pulling ffc1628 on mraspaud:add-spline-interpolation
into 5f44745 on pytroll:main.

[djhoese]

I think maybe I would have done some of the subclassing things a little different, but I don't feel strongly enough to spend the time to suggest something that would ultimately be equivalent. Looks good to me.