Refactor Optical flow interfaces

[dnerini]

Following #101, complete refactoring of the LK optical flow method.

The motion.lucaskanade module now only includes a wrapper to the original opencv method (lucaskanade.track_features) which returns sparse vectors and a the main dense_lucaskanade method which returns a motion field.

All others subroutines have been reorganized in few new modules:

• utils.interpolate module (RBF interpolation)
• utils.cleansing module (declustering, outlier detection)
• utils.images module (corner detection, morphological opening)

Corresponding documentation here:
https://pysteps.readthedocs.io/en/refactor_lk/

Note that this branch has been merged with the add_decorators branch.

[codecov]

Codecov Report

Merging #110 into master will increase coverage by 0.39%.
The diff coverage is 78.37%.

@@            Coverage Diff             @@
##           master     #110      +/-   ##
==========================================
+ Coverage   57.32%   57.72%   +0.39%     
==========================================
  Files          87       91       +4     
  Lines        6465     6753     +288     
==========================================
+ Hits         3706     3898     +192     
- Misses       2759     2855      +96

Impacted Files	Coverage Δ
pysteps/tests/test_nowcasts_steps.py	100% <ø> (ø)	⬆️
pysteps/motion/proesmans.py	88.88% <100%> (+7.63%)	⬆️
pysteps/tests/test_interfaces.py	99.12% <100%> (+0.04%)	⬆️
pysteps/utils/__init__.py	100% <100%> (ø)	⬆️
pysteps/utils/cleansing.py	58.58% <58.58%> (ø)
pysteps/utils/interpolate.py	68.25% <68.25%> (ø)
pysteps/tests/test_motion.py	91.39% <70.83%> (-7.2%)	⬇️
pysteps/motion/vet.py	84.45% <75%> (+0.67%)	⬆️
pysteps/utils/images.py	80.48% <80.48%> (ø)
pysteps/motion/lucaskanade.py	87.25% <90%> (+8.8%)	⬆️
... and 11 more

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[pulkkins]

Good work!