Remove map metadata from config object #30

I think this should not be part of the config at all. Possibly the config object could provide a method to access available maps, but this is just a shim put in place as long as InSEEDS and pycopanlpjml have not been updated to access the maps themselves.

Originally posted by @sieben-gea in #19 (comment)

As discussed in the pull request, the config object currently contains some metadata, in particular maps containing enum labels for decoding result datatsets. These should not be part of the LpjmlConfig class.

I propose a method like: config.read_meta_map("landuse"), which would read any available map from either the metadata JSON file or the legacy config.