Add mars request dispatch to data source

[cfkanesan]

Purpose

Add the ability to pass a mars request object to the data source retrieve method. This is a preliminary step for the mch model data module implementation

Code changes:

• data_source.DataSource.retrieve accepts an argument of type mars.Request
• The grib definitions context is no longer derived from the request model attribute. In practice, all defined models required the cosmo definitions as IFS grib data does not define the model mars key. Setting data_scope config to ifs to disable the cosmo definitions context remains possible.