Center of Gravity

[slobberingant]

Center of gravity is the average of the center of mass of each object multiplied by it's weight.

This issue is dependent on #598. A material system needs to be in place before we can calculate the weight and therefore impact of each part in an assembly on the center of gravity.

The impacts and scope of the material system are beyond my skills and knowledge but hopefully I can help create a center of gravity function.

My thoughts are to create a new enum CenterOf.GRAVITY. Then alter the Mixin3D center function to include a calculation for this.

I couldn't find an OCCT function for this so it may need to be implemented in Python.
I'm looking for some feedback on how to best approach this before I have a go with the code.