fix: beagle AP field

[nicholema]

This PR ensures that mergeSTR will output format fields from hipstr-beagle. Specifically, it adds support for the AP1, AP2, and DS fields.

Note that only hipstr input is supported for now. We did this because the imputation panels all seem to be produced by hipstr, anyway.

Checklist

• [x] I've checked to ensure there aren't already other open pull requests for the same update/change
• [x] I've prefixed the title of my PR according to the conventional commits specification. If your PR fixes a bug, please prefix the PR with fix: . Otherwise, if it introduces a new feature, please prefix it with feat: . If it introduces a breaking change, please add an exclamation before the colon, like feat!: . If the scope of the PR changes because of a revision to it, please update the PR title, since the title will be used in our CHANGELOG.
• [x] At the top of the PR, I've listed any open issues that this PR will resolve. For example, "resolves #0" if this PR resolves issue #0

• [x] I've explained my changes in a manner that will make it possible for both users and maintainers of TRTools to understand them

• [x] I've added tests for any new functionality. Or, if this PR fixes a bug, I've added test(s) that replicate it
• [x] All directories with large test files are listed in the "exclude" section of our pyproject.toml so that they do not appear in our PyPI distribution. All new files are also smaller than 0.5 MB.
• [x] I've updated the relevant REAMDEs with any new usage information and checked that the newly built documentation is formatted properly
• [x] All functions, modules, classes etc. still conform to numpy docstring standards
• [ ] (if applicable) I've updated the pyproject.toml file with any changes I've made to TRTools's dependencies, and I've run poetry lock --no-update to ensure the lock file stays up to date and that our dependencies are locked to their minimum versions
• [x] In the body of this PR, I've included a short address to the reviewer highlighting one or two items that might deserve their focus