Make libmdaxdr and libdcd cython classes use pxd format so they can be cimported

[hmacdope]

Is your feature request related to a problem?

If we want to make our xdr and dcd reading and writing libraries cython importable and public as discussed in #3734, the cython classes in the respective .pyx files need to have accompanying .pxd files that expose the implementation.

Describe the solution you'd like

The required pxd files should expose the cython datastructures.

Additional context

I will likely do this in a separate PR #3888 for the libdcd as its changes are targeted at 3.0, while for libmdaxdr this can be done in #3892.

[hmacdope]

EDIT: Change of plans I have "pxdified" libdcd in #3888

[hmacdope]

with the merge of #3892 this is now addressed.