Nick Hay

Bit of a commit dump. Major things are a total derivative operation and various new simplifications. Minor changes allow IPython visualization of Nodes, multidimensional numeric gradient, total derivative, more reproducible...

## HL7 FHIR Pull Request _Note: No pull requests will be accepted against `./source` unless logged in the_ [HL7 Jira issue tracker](https://jira.hl7.org/projects/FHIR/issues/). If you made changes to any files within...