Refactor importers

[dnerini]

The current importer module includes a fairly arbitrary list of importers that are specific to data formats used by various national weather services. This list cannot grow indefinitely each time a new use case appears.

Instead, we should leverage the recent option to use importer plugins and only keep importers that are general enough.

I therefore suggest to

• migrate importers that are specific to a national data format as separate plugins (hosted wherever it makes more sense, including the parent pySTEPS organization account).
• keep only importers that refer to internation standards, as for example the ODIM HDF5 format (see #214).

[RubenImhoff]

For the nwp_importers work in progress in the separate pysteps-nwp-importers repo. See: https://github.com/pySTEPS/pysteps-nwp-importers/pull/1