Edited HUfromCT.py

Initial start to add element set to part / assembly
Modified HUfromCT.py

Added creation of element set
Modified getHUfromCT to only open the CT slices that are required, not
the entire stack
Bug fixes for CT slice range
Merge pull request #5 from mhogg/Add_element_set

Update writeOdb to also create an element set of the part instance / assembly set