Make some dependencies optional

[sandorkertesz]

What maintenance does this project need?

Requirements file needs to be revised for earthkit-data as it installs too many things. We could solve that through optional dependencies in the toml config, for instance having an option to install all dependencies, or just a subset, or just the basic ones.

Organisation

ECMWF

[sandorkertesz]

At the moment, there are these sources requiring additional packages/libraries:

• "cds", "ads": cdsapi
• "mars": ecmwf-api-client
• "ecmwf-opendata": ecmwf-opendata
• "polytope": polytope-client
• "wekeo", "wekeo-cds": hda
• "fdb": pyfdb

We have two options to make any of them optional:

1. turn the source into a plugin. E.g. the "polytope" source can be implemented as the"earthkit-data-polytope" plugin. So it will only be available when the user installs it (and this will install all its dependencies)
2. make the dependency optional in the config and print a proper error message when someone wants to use a source where the dependency is not installed.

[sandorkertesz]

There is also the question of the supported data file formats. The following list contains the current formats and their (Python) dependencies:

"grib": eccodes, (cfgrib, xarray for to_array)
"bufr": pdbufr
"odb": pyodc, (pandas for to_pandas)
"netcdf": netcdf4, xarray
"csv": (pandas for to_pandas)
"geojson: (geopandas for to_pandas)

Please note that when xarray is used currently dask is required too.

[tlmquintino]

If we start by just making the dependency optional, I think it will be simpler/quicker.
Then later we can consider plugins, and separate repos etc.