add FDJUMP parameters to the designmmatrix

[siyuan-chen]

For the IPTA DR3 combination new parameters describing DM offsets between different PTA data sets have been introduced in tempo2: FDJUMP and FDJUMPDM
https://bitbucket.org/psrsoft/tempo2/pull-requests/84
and related pull requests in tempo2

This PR adds these FDJUMPX parameters to libstempo, such they are included in the designmatrix and can be passed to enterprise.
The structure follows the normal phase JUMP parameters with FDJUMP and FDJUMPDM parameters being called numbered FDJUMPX in libstempo.

[mattpitkin]

@siyuan-chen do you want to get this PR merged in before a new release is made? It's probably worth rebasing this branch with master now #69 is merged and making sure the CI tests now run.

[siyuan-chen]

yes, thanks for the reminder, Matt. Actually, this still needs some testing to be sure that it works properly with tempo2. So, not urgent for now.
As I mentioned in the other #69 , feel free to go ahead with 3.5.0. Once the PRs to add the new functionality from tempo2 are done, we can update to 3.5.X.

[mattpitkin]

I've released v2.5.0 on PyPI now and will update the conda installable version too.

[siyuan-chen]

Hi @mattpitkin , I think this PR is fine to be merged. (there is an error in one of the tests, I don't see why it happens though)

[mattpitkin]

I re-ran the test that failed and it seemed to work.

[siyuan-chen]

Thanks for having a look, Matt. We can have this on the master branch for a while and if there are no bugs found, we can make a version 2.5.1. And that should be the version for the IPTA DR3 work.

[mattpitkin]

I've merged this in now and am happy to wait and see if there are any fixes required before publishing a new release.