SCMI Writer can produce un-ingestable AWIPS files

[djhoese]

Describe the bug

This was reported to me by @spruceboy as part of a Polar2Grid bug. By default, the SCMI writer will use the hostname of the current machine as the "production_location" global attribute in the produced NetCDF4 files. For some systems this value can be really long. AWIPS, the software ingesting these files, has an internal database limit of 31 characters (as far as we know) for this field.

Expected behavior
AWIPS can read any file generated by the SCMI writer.

Actual results
AWIPS ingest fails with an odd error. Note, this can be worked around by specifying the ORGANIZATION environment variable.

Proposed solution
Raise an error if the product_location is longer than 31 characters. The error message should include a mention about the environment variable to override the value.

Other solution would be to just always limit the string to 31 characters, but I think this is a little too magical and could not be what the user wants. It would also be nice if production_location could be provided from the python code (no environment variable needed).

FYI to anyone who fixes this before me, the place to look is satpy/writers/scmi.py:AttributeHelper._global_production_location().