Specifying desired sites for IGWN (OSG) jobs

Assuming it does not already exist, this is a feature request for support for desired/undesired sites for "OSG" jobs in bilby_pipe.

It is sometimes useful to specify the site(s) at which jobs should or should not run when using distributed resources in the IGWN grid (loosely referred to as the OSG). A good example is the current need to use ComputeCanada resources.

To do so, submit files require:

requirements = IS_GLIDEIN=?=True
+DESIRED_Sites = "site_name1,site_name2"

where the requirement may be appended with && (IS_GLIDEIN=?=True) if any already exist [1].

Similarly, for +UNDESIRED_Sites to exclude certain locations.

Note: the desire/undesired commands are mutually exclusive; specifying both may have unpredictable behaviour (and I struggle to think of why anyone would do so on purpose).

[1] The +DESIRED_Sites command is only recognised by glidein jobs; it will be ignored by jobs in the local pool.