While here, add a symlink to test_data if the build directory is not the
root one

[SMALL IMPROVEMENTS] When solving dual LP switch infeasible and unbounded return flags. In all equilibrium algorithms check whether the number of contacts is 0, in which case return appropriate result (typically infeasible). Improve test_static_equilibrium.

[IMPORTANT CHANGE] Take com as a 3d point rather than a 2d point (this should allow to consider cases in which gravityis not aligned with the z axis in the future).

Assume all contacts have same friction coefficient (change signature of setNewContacts method). Convert b0 (the minimum contact force generator coefficient) to e_max (the norm of the minimum force error that results in a violation of the friction cones).

Return the status of the LP solver rather than returning a boolean, so that the user can have a better understanding of what happened when the LP could not be solved.

Add new LP problems in test_data folder to use them in test_LP_problem. Change range of admissible pitch-roll angles for random contact generation from 0 to twice the friction cone angle. Clean code.

Add binary files describing some LP problems in folder test_data. Add test LP solver using binary files.

Extend code to handle arbitrary number of generators per contact. Minor improvements here and there.