Rewrite interpolation structure to utilize extendible class structure

[RuneDominik]

While the current structure of pyirfs interpolation functionalities might suffice with only few interpolation algorithms implemented, extending it with further algorithms (e.g. Moment-Morphing and extrapolation) would result in at least some I/O-Code doubling. This PR refactors the current interpolation algorithms into a class-structure with the upside of having:

• Somewhat defined in- and output so algorithms can simply be exchanged
• Predefined data sanity-checks (e.g. not each interpolation method has to check if the target-point lies within the grid)
• Predefined "interface" to use future extrapolators

[codecov]

Codecov Report

Base: 90.71% // Head: 92.43% // Increases project coverage by +1.72% 🎉

Coverage data is based on head (e2af267) compared to base (a3e810b).
Patch coverage: 99.22% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #210      +/-   ##
==========================================
+ Coverage   90.71%   92.43%   +1.72%     
==========================================
  Files          42       49       +7     
  Lines        1723     1996     +273     
==========================================
+ Hits         1563     1845     +282     
+ Misses        160      151       -9     

Impacted Files	Coverage Δ
...irf/interpolation/tests/test_base_interpolators.py	98.29% <98.29%> (ø)
pyirf/interpolation/base_interpolators.py	98.52% <98.52%> (ø)
pyirf/interpolation/__init__.py	100.00% <100.00%> (ø)
pyirf/interpolation/griddata_interpolator.py	100.00% <100.00%> (ø)
pyirf/interpolation/interpolate_irfs.py	100.00% <100.00%> (ø)
pyirf/interpolation/quantile_interpolator.py	100.00% <100.00%> (ø)
.../interpolation/tests/test_griddata_interpolator.py	100.00% <100.00%> (ø)
pyirf/interpolation/tests/test_interpolate_irfs.py	100.00% <100.00%> (ø)
.../interpolation/tests/test_quantile_interpolator.py	100.00% <100.00%> (ø)
pyirf/conftest.py	93.75% <0.00%> (+43.75%)	⬆️

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[maxnoe]

Looks mostly good, some comments inline.

Due to the changes, the top-level API (the functions in interpolate.py) now seem all untested, which is not good, that should be fixed.

[maxnoe]

The only thing I don't quite like yet is the structure. Having submodules interpolation and interpolators right next to each other is confusing.

I'd create a single submodule interpolation (directory) and move the interpolation.py to something like interpolation/interpolate_irfs.py and then import those and the other classes in interpolation/__init__.py, that way, the imports also stay backwards compatible with what is currently in interpolation.py.

[maxnoe]

objects → classes, scripts → functions?

[maxnoe]

unused import