Read MSG HRIT using satpy to generate intermediate CF-responsive netcdf files

This adds an MSG reader that uses URI-format input names in the form satpy-cf:GLOB:CHANNEL e.g. satpy-cf:/data/test/input/msg/msg-example/H-*202111090900*:IR_120 .

It shells out to a python3 interpreter which is assumed to include satpy and the necessary decompression library. This invocation must run without error to have a valid environment for this:

${SATPYCF_PYTHON:-python3} -c 'import satpy, pyPublicDecompWT, netCDF4

Caveats: It currently only supports radiance queries from the MSG HRIT output, and center wavelength and channel number translation is not yet included.