Standardize platform and sensor names

[djhoese]

This is in addition to #749.

There is a bit of inconsistency between satpy and pyspectral about what the name of a platform or sensor should be. We also decided early on in Satpy that composite filenames would be based on the sensor name and lowercased all of them. So this means we have readers like "mersi2_l1b", metadata with sensor "mersi-2" and platform names of "FY-3D", composite configs named "mersi-2.yaml", and pyspectral preferring the WMO OSCAR name of "MERSI-2".

As part of 1.0, we should break backwards compatibility and standardize all of this so things are consistent and follow some official resource for naming.

[BENR0]

See also #3008 (and also #2994)