ENH: update versioning/release

[kmuehlbauer]

resolves #263

With these changes the git_revision will be build from the git describe command:

X.Y.Z-N-g0123456

• X.Y.Z - latest git tag
• N - commits since latest tag
• g0123456 - short git hash

With N = 0 we have a tagged release, with N > 0 we have devel. In the version.py is written:

• short_version - X.Y.Z
• version - X.Y.Z or X.Y.Z-N-g0123456 (depending on release or not)
• full_version - X.Y.Z-N-g0123456
• git_revision - long git hash
• release - True or False

If the the source is no git-repo and the version.py is missing, then pre-defined values ('unknown') are written to version.py.

Overall this has two benefits:

1. We have a more meaningful full_version as described above
2. No need to set ISRELEASED before and after cutting release

[codecov]

Codecov Report

Merging #265 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #265   +/-   ##
=======================================
  Coverage   91.54%   91.54%           
=======================================
  Files          30       30           
  Lines        4517     4517           
=======================================
  Hits         4135     4135           
  Misses        382      382

Flag	Coverage Δ
#wradlibtests	91.54% <ø> (ø)	⬆️

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