CDFs and netCDFs from SPDF/CDAWeb master metadata override

[candeynasa]

As mentioned at the 2019 Fall PyHC meeting, SPDF uses metadata-only CDFs for adding/improving the metadata in the original CDFs and netCDFs. The original files are available via HTTPS https://spdf.gsfc.nasa.gov/pub/data/. Most datasets in CDAWeb have improved (in some cases, greatly improved) metadata in Master CDFs https://spdf.gsfc.nasa.gov/pub/software/cdawlib/0MASTERS/. When reading the data files, the Master CDF (if available) should be opened first for its metadata. Alternatively, and generally better, is to use CDAWeb REST web services (CDAS) to create an output CDF that has the specific time range and variables desired https://cdaweb.gsfc.nasa.gov/WebServices/REST/, as called in ai.cdas or some test code https://test.pypi.org/project/cdasws/ based on SpacePy.