BulletDskShape doesn't properly work with multi-segment DSKs

[jessemapel]

ISIS version(s) affected: 3.7.0 (pre-release)

Description

The methods in BulletDskShape only access the first segment.

How to reproduce

The unit test it self has issues. For the multi-segment test, it should have 26 vertices and 28 plates.

This is primarily present in BulletDskShape::getNumTriangles, BulletDskShape::getNumVertices, and BulletDskShape::getTriangle

Possible Solution

We need to iterate over all the segments when counting things. We also need to determine how we want BulletDskShape::getTriangle to work. We could implement a linear indexing where the indexing for each segment starts after the end of the previous. Another option would be indexing by both the segment index and the plate index.