improvement projection (#379)

* ENH: georef.projection
* STY+FIX: use black to format georef/projection.py and fix flake, use LooseVersion
* MIN: finalize PR

Co-authored-by: Edouard Goudenhoofdt <egouden@users.noreply.github.com>
Co-authored-by: Kai Muehlbauer <kmuehlbauer@users.noreply.github.com>

ENH: improvement ipol (#390)

* improvement ipol
* FIX+STY: refactor new Ipol-classes, use pytest in ipol-tests, use black/isort
* DOC: update docstrings
Co-authored-by: Edouard Goudenhoofdt <egouden@users.noreply.github.com>
Co-authored-by: Kai Muehlbauer <kmuehlbauer@users.noreply.github.com>

FIX: introduce 'call_kwargs' in comp.togrid (#425)

* FIX: introduce 'call_kwargs' in comp.togrid to parametrize interpolator call
* STY: use black to format comp.py

ENH: read ODIM/GAMIC RHI (#424)

* ENH: derive scantype (PPI/RHI) automatically (works for GAMIC, should work work ODIM, if standard compliant)
* STY: use black to format io/xarray.py
* STY: use black to format test_io_odim.py
* FIX: correct __repr__

MIN: use 'cov_nan' as standard NaN handling method2 in util.derivate (#423)

Merge pull request #422 from kmuehlbauer/rework-vulpiani

Rework Vulpiani PhiDP/KDP Processing

MIN: adapt flake8 noqa

STY: assume black standards when testing with flake8

STY: use black to format dp/util/dp-tests

ENH: add dp-tests

ENH: update process_raw_phidp_vulpiani (add threshold keyword args, use scipy.integrate.cumptrapz instead of numpy.cumsum)

MIN: add finite difference scheme as described in Vulpiani2012

MIN: fix docs in util.despeckle

ENH: update unfold_phi_vulpiani

ENH: update kdp_from_phidp

ENH: update util.derivate

ENH: speedup `unfold_phi_vulpiani`, fix tests (#421)

ENH: speedup `despeckle`, move to utils, deprecate (#420)

ENH: improve derivation function (#419)

* ENH: improve kdp derivation performance, break out derivation code to module `util` for better reusability
* MIN: appveyor matrix, add python 3.8
* ADD: calculate slope using covariances
* WIP: more performance
* WIP: refactoring, docstrings, tests

FIX: correct padding and correct nan-filling for multidimensional arrays in `dp.texture` (#418)