refactor impulses

[michguo]

• Originally, the LCP solution was list of pointers. Each pointer pointed to a scalar py. We'd increment the pointer to access px and pz in the presence of friction.
• Now, the LCP solution is a matrix with shape (numContacts, 3). The last two columns of each row are only used if friction is on.

TODO: Some tests are failing. To pass the tests, we need to add support for non-contact constraints with dynamic size impulses vector.