improve nc4 support in data_model

[jmccreight]

currently many data_model and DatasetDict tests are not carried out using netcdf4, but only xarray. this is because certain variable types (particularly time) needs extra code. I explored the code for how xarray solves this (using netcdf4 as a backend), but it is not obvious.

along these lines, the test_prms_param_separate.py does not separate parameters using netcdf4. Would be good to have that included.