Add metadata handling object

[sandorkertesz]

Relates to #146

Still work in progress but there is a basic implementation.

The implementation contains the base class Metadata and the following derived classes:

• RawMetadata
• GribMetadata

See the notebook for examples: https://earthkit-data.readthedocs.io/en/feature-metadata-object/examples/metadata.html

The new classes Field and FieldList provide abstraction for fields and fieldlists. The GRIB implementation and the newly created numpy field + fieldlist are based on these (the NetCDF fieldlist is still to be redesigned):

• Field is a base class for: GribField, NumpyField
• FieldList is a base class for: GribFieldList, NumpyFieldList

NumpyField is a combination of a numpy array and a Metadata object, while NumpyFieldList is a sequence of these fields. A NumpyFieldList can be created with FieldList.from_numpy(). It has an experimental write support.

The following example shows how to create a NumpyFieldList from the results of field based computations: https://earthkit-data.readthedocs.io/en/feature-metadata-object/examples/numpy_fieldlist.html

[oiffrig]

Thanks! Looks good so far, I'll try it out