Add data_source module

[cfkanesan]

Purpose

The goal is extract the logic regarding the fetching of data from the construction of the xarray dataset. The DataSource class takes over the responsibility of:

• loading the appropriate GRIB definitions based on the data_scope with a fallback on the request model field
• dispatching the request to either given datafiles or an FDB instance
• injecting contextual values into the request

Driving the data loading based on the product descriptions will be handled in a future PR.

Code changes:

• Add data_source module
• Breaking: removed the load_fields method from the GribReader class
• Breaking: the load method of GribReader class now takes a mapping of labels to requests and returns a mapping of labels to xarray.DataArray

[victoria-cherkas]

Looks great! (minus pre commit fail)