Write multi-conformer PDBS with RDKit

[jthorton]

• Fix RDKit cannot write multi-model PDB files #568
• Add tests
• Update docstrings/documentation, if applicable
• Update changelog

This should fix RDKit and allow it to write multi-conformer PDB files, also added a test for this.

[jthorton]

This has made me realise I don't think we test all of the possible file output formats which I could add test for here if required?

[mattwthompson]

Which formats are you referring to? We are in the process of adding some in #569

[jthorton]

Ahh yes I mean backend toolkit formats more like PDB and SDF I know SDF is more heavily tested now that the property PR is merged.

[j-wags]

This has made me realise I don't think we test all of the possible file output formats which I could add test for here if required?

I'm starting to appreciate how "output formats" are complicated. That is, writing to a particular output format isn't just "making a valid file within that format's specification", but rather "deciding on exactly which characteristics of a molecule we guarantee to preserve in a round trip to that format".

So, this doesn't mean "don't add tests for new formats" -- after all, we do "support" them in the API, and testing is better than no testing. I mean something more like "don't be surprised if this task is a rabbit hole, and we end up with a big table of what we do and don't preserve through trips to different formats"

So, this PR shouldn't add tests for every output format, because it will take a long time.

[j-wags]

Thanks for updating the SDF tests to use StringIO -- That seems like a better practice, I was just super lazy when I first did it.

This PR looks good. Thanks for adding a test for it as well.

[j-wags]

Oh, and please update the release notes before you merge!!

[jthorton]

Good point I had a feeling it may be a bigger job than I expected Ill get this finished up today.