Add metadata handling object

[sandorkertesz]

earthkit-data objects are immutable but when we perform computations we need to create new objects from existing ones. The computations in earthkit-meteo will always take numpy input and result in numpy output.
The following code snippet illustrates a possible workflow that:

• starts from an earthkit-data object
• performs the computations in earthkit-meteo using numpy arrays
• generates a new earthkit-data object using the resulting array and the metadata from the original object

#ensemble is a GRIB fieldlist with 51 fields
np_efi = earthkit.meteo.efi(ensemble.to_numpy(), clim)

# option 1
metadata = ensemble[0].metadata().override(param=‘efi’)

# option 2
metadata = earthkit.data.Metadata(param=efi_param(ensemble[0]), template=ensemble[0])

fs_efi = earthkit.data.FieldList.from_numpy(np_efi, metadata)
fs_efi.save(…)

The key element here, regarding the current issue, is the metadata object. #107 provides guidelines for the concrete API it should implement.