Undesirable "merge" behavior in MAVEN and GOES load routines

[jameswilburlewis]

When loading MAVEN data, load_data iterates through the CDF or STS files returned by the SDC request, then loads them one by one with merge=True. This is probably not the best implementation, since users will probably expect existing tplot variables to be replaced, not appended, when a new request is being processed.

Similarly, in the GOES load() and loadr() routines, NetCDFs are loaded one-by-one with merge=True.

It would probably be better to process the entire list of CDF, NetCDF, or STS filenames in a single call, with merge=False as the default.

See also: #911