Add interface for custom netcdf forcing files

[JaroCamphuijsen]

Implementation in https://github.com/omuse-geoscience/omuse/pull/87

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The ERA5 interface enables us to download ERA5 data as netcfd files and access them as a regular model grid inside OMUSE. However more flexibility is required:

• data can come from other sources than CDS, or
• some custom made preprocessed data might be used
• different file types might be required as forcing

This is currently not possible in the ERA5 interface.

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Proposal Set up a new interface, perhaps called "data" or "forcing" which at least accepts netcdf grid files, but ultimately can also be extended to other file types. It should function in a similar way as, and can be based largely on the ERA5 interface and perhaps at some point replace the ERA5 interface, however it should take a path to one or multiple netcdf files as argument. In the future we could even merge the new interface and the old ERA5 interface and automatically downloading the data from CDS could be an option in the new interface.

[JaroCamphuijsen]

Basic interface is there in pr https://github.com/omuse-geoscience/omuse/pull/87. Additional enhancements can be suggested in issue https://github.com/omuse-geoscience/omuse/issues/94.