add ability to index datasets with no time axis to index_netcdf

[corviday]

We would like to be able to index some raster files that give time-independent information about a watershed, such as elevation or drainage data. These would be used by the PCEX backend to calculate watershed data for a watershed API endpoint.

Currently, index_netcdf explicitly requires all datasets to have a time axis. index_netcdf should instead allow the indexing of datafiles that meet both the following requirements:

• frequency metadata attribute is set to the CF Standard fx ("fixed")
• no time dimension or variable at all

as indexed datasets with null data_files.time_set_id and data_files.t_dim_name in the modelmeta database.