Add Moment Morph Interpolation

[RuneDominik]

This is a new version of #221 that gets rid of most classes. It thus supersedes #221. @maxnoe's comments in #221 were included in this version

This PR introduces another interpolation method (Moment Morphing, see [1]). While performing comparably to the already existing Quantile Interpolation the main points in favor of this method is that it can be can be separated into two steps. First, one has to compute interpolation coefficients and then these are used to morph and interpolate the input pdfs. When changing the coefficients (e.g. changing interpolation to extrapolation or changing from interpolation in a triangle to interpolation in a rectangle) the second part remains unchanged. Consequently, using this for extrapolation is quite simple.

This PR is quite extensive in its additions. Most of these are actually tests, checking all kind of broadcasting and border cases.

[1] M. Baak, S. Gadatsch, R. Harrington and W. Verkerke (2015). Interpolation between multi-dimensional histograms using a new non-linear moment morphing method. Nucl. Instrum. Methods Phys. Res. A 771, 39-48. https://doi.org/10.1016/j.nima.2014.10.033

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.94 🎉

Comparison is base (489e954) 92.59% compared to head (b3839f1) 93.53%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #229      +/-   ##
==========================================
+ Coverage   92.59%   93.53%   +0.94%     
==========================================
  Files          50       52       +2     
  Lines        2120     2429     +309     
==========================================
+ Hits         1963     2272     +309     
  Misses        157      157              

Impacted Files	Coverage Δ
pyirf/interpolation/__init__.py	100.00% <100.00%> (ø)
pyirf/interpolation/moment_morph_interpolator.py	100.00% <100.00%> (ø)
...erpolation/tests/test_moment_morph_interpolator.py	100.00% <100.00%> (ø)

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[maxnoe]

This looks good aside from readability in the tests, would be great if the test case data would be easier to read, e.g. by factoring out some functions to reduce the nesting levels.