Using NIDM acronym in NIDM-Results specs + UML chart updates

This pull request updates the NIDM-Results specification to:
-  use "NIDM" acronym as requested in incf-nidash/nidm#111.
- include newer versions of the UML class diagrams.
- include various small fixes.

@dbkeator: if you plan on working on the NIDM-Results spec, could you please make sure you start from this newer version? Thank you!