XrootD class in se.py broken and incomplete

[hannahbnelson]

TheXrootD class in se.py is incomplete.

Compared to the Local class, it is missing a permissions function, which means that if XrootD is used as the first option for the output storage object in a lobster config, the validation of the workflow fails. At minimum this is because there is no permissions function, which is called in StorageElement.makedirs().

This means that currently, users have to list some local file access (eg. file:///cms/cephfs/data/store/) as an output option so that the validation can pass, even if they know they will not be able to use it for their actual jobs (eg. when running on opportunistic resources, ceph is not accessible via direct file access but currently needs to be listed in the output).

The XRootD class might be missing more components, but it's unclear since lobster is missing this function and this is what breaks first.