BUG: gnss_tec cannot load for xarray 0.18.0

[jklenzing]

Loading vtec data into xarray is not working with 0.18.0. Test code (assuming data has been downloaded):

import datetime as dt
import pysat
import pysatMadrigal

test_date = dt.datetime(2017, 11, 19)
gns = pysat.Instrument(inst_module=pysatMadrigal.instruments.gnss_tec, tag='vtec')
gns.load(date=test_date)

Error message:

~/code/core/pysatMadrigal/pysatMadrigal/instruments/methods/general.py in load(fnames, tag, inst_id, xarray_coords, file_type)
     92         # Xarray natively opens netCDF data into a Dataset
     93         if len(fnames) == 1:
---> 94             file_data = xr.open_dataset(fnames[0])
     95         else:
     96             file_data = xr.open_mfdataset(fnames, combine='by_coords')

/opt/anaconda3/lib/python3.8/site-packages/xarray/backends/api.py in open_dataset(filename_or_obj, engine, chunks, cache, decode_cf, mask_and_scale, decode_times, decode_timedelta, use_cftime, concat_characters, decode_coords, drop_variables, backend_kwargs, *args, **kwargs)
    483 
    484     if engine is None:
--> 485         engine = plugins.guess_engine(filename_or_obj)
    486 
    487     backend = plugins.get_backend(engine)

/opt/anaconda3/lib/python3.8/site-packages/xarray/backends/plugins.py in guess_engine(store_spec)
    110             warnings.warn(f"{engine!r} fails while guessing", RuntimeWarning)
    111 
--> 112     raise ValueError("cannot guess the engine, try passing one explicitly")
    113 
    114 

ValueError: cannot guess the engine, try passing one explicitly

Discovered during work on #45. Previous CI tests using conda are still using xarray 0.17.0.

[jklenzing]

This seems to just affect gnss_tec, which uses the netcdf4. JRO still uses hdf5, which passes through correctly.

[aburrell]

Requires engine="netcdf4" for NetCDF4 files calling xr.open_dataset