h5netcdf read

[davidhassell]

Fixes #285 and #286

• Implements reading of netCDF files with h5netcdf, which also allows files in S3-compatible storage to be read.
• Implements netcdf_flattener in-house, rather than with an import. The new flattener can flatten files opened with h5netcdf, can flatten all CF references, and is more extensible to future CF references.
• Creates netcdf_indexer (based on netCDF4 functionality) that allows orthogonal indexing on h5netcdf variables.

[sadielbartholomew]

Thanks for addressing all of my feedback. I've done a final sanity check and it is all good. Great stuff, please merge!

[davidhassell]

Thanks again for your customary careful and detailed review - merging now ....