check_jumps needs to be rewritten for IPTA use

[rossjjennings]

Currently check_jumps doesn't know what to do with JUMPs that aren't NANOGrav-style -fe jumps. We should fix that.

[cuantar]

requires someone with knowledge of how the jumps work

[rossjjennings]

This could still be a useful enhancement, but it seems like it might be more trouble than it's worth in the IPTA context. For DR3, the approach so far has been to avoid the check_jumps() function altogether, which puts the burden of checking that jumps make sense on the person doing the combination. This is hard to avoid because different PTAs' systems of jumps are so different, and no one seems to have the complete picture.