Fix CLI `omero group perms` subcommmand

Propagate the recent changes made to the group arguments using --id/--name
options to the perms() method using parse_groupid() to return the group and
group identifier.

To test this PR, try out the various signatures of the `bin/omero group perms` subcommand, i.e.

```
bin/omero group perms --id 4 --type read-only
bin/omero group perms --name private-1 --perms 'rw----'
bin/omero group perms --id 4 --perms 'rwr---'
bin/omero group perms --name private-1 --type read-only
```

and check the permissions are updated using `bin/omero group list`

/cc @ximenesuk