Fix _post_process() in FDSNFactory

The FDSNFactory was not working with input streams that contained multiple
segments. This was because FDSNFactory wasn't doing the full "_post_process"
like, for example, EdgeFactory. More specifically, FDSNFactory's `_post_process()`
method did not convert masked arrays (which is how obspy prefers to deal with
gappy data) into regular numpy arrays with NaNs for missing data, as expected
in all geomag-algorithms code.

This became quickly obvious once some of the less stable ASL variometers
were processed, like US-EYMN. This fix should allow us to now pull and plot
EYMN, DGMT, and other stations that were not working with the previous
release of FDSNFactory.