Make a multiselect request to underlying seedlink client
Multiselect string is one or more comma separated
network/station/location/channel combinations as defined by seedlink
where location+channel may contain ‘?’ characters but should be exactly
5 characters long.
Abstract classes can override this to customize issubclass().
This is invoked early on by abc.ABCMeta.__subclasscheck__().
It should return True, False or NotImplemented. If it returns
NotImplemented, the normal algorithm is used. Otherwise, it
overrides the normal algorithm (and the outcome is cached).