Creating datasets.xml blocks for ERDDAP

[rsignell-usgs]

We should add a standard method for generating blocks for ERDDAP in each DSG class. ERDDAP's provided scripts take an abstract approach so they can support a wide variety of data, but since we already know the featureType and details of the dataset, generating the correct block should be easy.

[rsignell-usgs]

Some of my findings so far:

• The featureType=timeSeries netcdf files we produce have needed the following fixes to work in ERDDAP:

<dataset type="EDDTableFromNcCFFiles" datasetID="phsen_2ac1_efcb_5466" active="true">
...
 <att name="cdm_data_type">timeSeries</att>
 <att name="cdm_timeseries_variables">feature_type_instance, longitude, latitude</att>
 <att name="subsetVariables">feature_type_instance, latitude, longitude</att>
...

• To represent multidimensional variables as grid, we just need cdm_data_type=Grid:

<dataset type="EDDGridFromNcFiles" datasetID="phsen_2e2d_a110_30fa" active="true">
...
 <att name="cdm_data_type">Grid</att>
...

[abkfenris]

I've started a library to work towards generating ERDDAP dataset blocks.