Support pdbML

[douweschulte]

As pdbML is also an option given by rcsb.org it would be really neat if pdbtbx can also support this format. One caveat however is that #99 should be fixed first as all files are only given in zipped format. It seems to be directly related to the mmCIF format, so it should not be extremely hard to build in, here is the specification.

[OWissett]

Do we want to have this added for v0.12 or v0.13?

[douweschulte]

To be honest I just wrote the issue because I found out it existed. I do not know if anyone uses this file type. If there is usage and there is interest to implement support for it it can be part of either of the two updates if it is implemented in time.