Wradlib

- FIX: correct calculation of angle resolution in ODIM/GAMIC xarray readers
- FIX: update dim0-retrieval

fix: do not restrict variables, but read all variables for Cf/Radial1 data

fix OPERA/ODIM reader to read non standard files (use default values for ODIM/OPERA what-group items if they are not available)

Bugfix Release:

- ensure numpy type in GAMIC reader
- delete time "comment" only if needed (CfRadial), add moments
- add get_attr and get_attrs to RadarVolume for easy access to groups
- minor fixes to README.md and test requirements

- tested with a build matrix for Python 3.7, 3.8 and 3.9 under conda-forge channel environment on linux OS (GitHub Actions)
- tested with Python 3.9 under conda-forge channel on MacOSX (GitHub Actions)
- tested with Python 3.9 under conda-forge channel on Windows OS (GitHub Actions)
- tested wradlib-notebooks creation for linux OS (GitHub Actions) with Python 3.9

- compatibility with xarray 0.17.0
- minor fixes

- new CI using GitHub Actions
- tested with a build matrix for Python 3.7, 3.8 and 3.9 under conda-forge channel environment on linux
- tested with Python 3.9 under conda-forge channel environment on macos and windows
- testes wradlib-notebooks with Python 3.9 on linux os