Calculate inverse of mass matrix recursively.

[jslee02]

To handle contact/collision, we need inverse mass matrix of the system. Currently, we calculate mass matrix with O(n^2) performance and invert the mass matrix which takes O(n^3) computation time in general. Instead of directly inverting mass matrix, we will implement a recursive algorithm for the inverse mass matrix with O(n^2) computation time.

[jslee02]

Implemented this by 5070900