Advanced key join use case

[astrofrog]

Here is a use case discussed today in the context of OIFITS data (but would also be valid for UVFITS). We have a table of telescopes:

ID  Name
1   Telescope1
2   Telescope2
3   Telescope3

and then a list of points which each are associated with two telescopes:

ID1 ID2 Data
1   2   4.5
1   3   5.5
2   3   2.2

When selecting a row in the second table, it would be great to select all items in the first table that have a matching ID. I think this could probably be done with some advanced version of join_on_key.

[astrofrog]

I've added support for this in #974