Positional tracker support

[Stermere]

Adds an inverse kinematics solver to the skeleton allowing for positional and rotational trackers to be used together. An example would be using 3 positional trackers for the waist and feet and 2 rotational trackers on your leg to add knee-tracking. That is just one example though, any combination of trackers is theoretically supported. Needs SlimeVR/SolarXR-Protocol#139 merged before this can be merged.

[ImUrX]

why does constraint need to be abstract? why is there so many vars instead of vals? For example does allowModifications need to be modifiable?

I would very much like to reduce the OOP-style of this

[Stermere]

Constraint is abstract because different joints have different constraints, originally I wanted to make a hinge joint for knees and elbows but ended up not getting to that, there are still different constraint types though for example the CompleteConstraint is for bones that the solver should ignore, I don't know how I would do this without an abstract class but I am open to suggestions! With the current way things are set up the Var's cannot be changed to Val's without significant change, probably pulling the Constraint out of the Bone class, and moving it into the IKChain's. Let me know if that change sounds reasonable, and if you still don't like the abstract class what do you suggest I do to remove it?