Correct documentation for Stations.inventory

[kaiserls]

I noticed two minor things in the documentation of the Stations.inventory method, which could need correction / clarification:

• The documentation states that the argument required may be a list. However the type hint in the code does not include list and the code also checks for isinstance(required, tuple) and not for list. This could be corrected.
• And is there any reason why granularity="monthly" is not allowed/listed? Currently only the granularity 'hourly' or 'daily' are listed in the documentation.