return LcpResult in solveLcp

[michguo]

Defines a struct that solveLcp() returns called LcpResult, and stores all useful information from solveLcp() inside of this struct.

Notes:

• This branch should only be merged after merging in refactor impulses #72. The reason is that refactor impulses #72 changes the type of impulses from a vector to a matrix, and this PR assumes that the type of impulses is a matrix. Without merging in refactor impulses #72, your code will not build successfully.