Refactored GJK, EPA and MPR as a template class, reducing/removing the dependencies
to the rest of the Bullet code base. This code will replace the original GJK/EPA
in a future commit.

Added btMprPenetration, an implementation of Minkowski Portal Refinement
by Daniel Fiser. Original MPR idea is by Gary Snethen, and the first
implementation is here: https://github.com/erwincoumans/xenocollide
It is an alternative to EPA, although computing the local penetration depth.
EPA computes the global penetration depth. In many cases, MPR is sufficient
and performs better than EPA.