Adds new nurbs format

[purepani]

Pull request header

Adds the new nurbs format used by xcat v2. This automatically detects whether the file is the old or new format.
I haven't tested this yet beyond making sure that xcist runs(I haven't looked at the resulting cts), so testing would be appreciated; I might be able to manage that later, but I've been short on time recently, so perhaps not.

Pull request description

• Updated version in setup.py and catsim/init.py

[purepani]

Just to add some details,
The MU_ID field in the surface struct is changed into an array of a new struct MU containing the fraction and ID. It parses the old format with the mu id and an assumed fraction of one.

Attentuation is calculated by a weighted addition of the material attenuation.

Also, there was a seemingly unused function that I commented out since I didn't immediately see how to adapt it, and it was unused as far as I could tell. If you could confirm, I can also just remove the function.

[purepani]

As a note, I believe this is functional, but I'd test it before merging @MingyeWu