MAINT: pysat 3.2.0 compatibility

[jklenzing]

pysat 3.2.0 changes how parse_fixed_width_filenames works. For an empty fileset, values are returned as None rather than an empty list. This is a breaking change for the cosmic_gps instrument.

Since len cannot be run on NoneType, the check for files is updated to

stored['year'] is not None and len(stored['year']) > 0

where the first bit is the pysat 3.2.0 version, and the second bit is the pysat 3.1.0 check. When 3.2.0 is released, this can be simplified and a version limit can be added.

[jklenzing]

There is an additional issue with pysat 3.2.0 tests failing on github actions. The tests are killed prematurely.

Cannot test this locally, as tests are killed for any pysat version prematurely.