Check Asana task #2

- set 'z' to required in Heatmap and Contour https://app.asana.com/0/11724913287040/14477880045186
- fill in 'traceref' description https://app.asana.com/0/11724913287040/14477880045210
- correctly describe current state of 'domain' in 'angularaxis' and 'radialaxis' https://app.asana.com/0/11724913287040/14477880045222
- add info to README.md about artifacts