Add bp_parse_args filter to groups_get_groups

[rekmla]

This would make it possible to avoid plugin modifications (bp-event-organiser for example) when group filtering is required. This can currently be accomplished before the query results are returned with apply_filters_ref_array( 'groups_get_groups', array( &$groups, &$r ) ). Something more efficient would be desirable.

[slaFFik]

bp_parse_args() in that function (imo not in BP_Groups_Group::get()) should be used there instead of wp_parse_args() with 'get_group' filter key, I suppose.

Related and fixed: #7250

[djpaul]

In 11286:

Groups: use bp_parse_args in groups_get_groups.

This gives us a new filter that can be used to modify query arguments
before passing it to BP_Groups_Group::get.

Fixes #7327

Props slaFFik, rekmla