Update initialisation

[nils-herrmann]

The current implementation uses one key for all the API endpoints. Springer Nature changed its authentication and now users get an api key for Meta and OpenAccess (Metadata was discontinued). sprynger should now be initialised with api_key_meta or api_key_oa or api_key. sprynger should use the corresponding key when making the requests. Examples of valid initialisations:

from sprynger import init

init(api_key ="def")
init(api_key_meta ="def", api_key_oa="abc" )
init(api_key ="def", api_key_meta ="def", api_key_oa="abc" )

Update documentation (and Redme). Mention that metadata is being discontinued and raise a warning when used.
Update tests.