Arbitrary pyramidal approximation

[Daniel K. O.]

Since some people are willing to sacrifice some performance for greater accuracy, the contact joint should accept setting the number of friction directions. This allows for an arbitrary approximation of the friction cone.

KODE implements this feature, it can be used for reference: http://bitbucket.org/danielko/kode/src/default/src/joints/Contact.cpp?at=default

Note that the mu, rho and slip coefficients must be scaled down by the apothem of the n-sided polygon, since that's how much "wider" the pyramid becomes.