Issue 396 - Migrate to ClinVar XML V2

[apriltuesday]

Closes #396

• Parse XSD version from header so we can continue to parse historical V1 data
• Extract clinical significance info from the ClinVarRecord class into a new ClinicalClassification class
  • This is not really necessary right now, but enables us to model the additional structure more completely in the future.
• If there is a single clinical classification in an RCV and a single description for that classification, this is consistent with the current schema so we proceed as usual.
• If there are multiples, this is logged and counted as a skipped record, but will not crash the pipeline.
• Most of the other changed files are test datasets; for these I used the same RCVs but from a recent ClinVar release to keep things as stable as possible, but of course there are still differences...