Add comments to nonlinear solver code

[maxpaik16]

Rename compute_collision_free_step_size to compute_max_step_size

Add doxygen style comments to the following headers:

polysolve/nonlinear/Problem.hpp

polysolve/nonlinear/Solver.hpp

polysolve/nonlinear/descent_strategies/DescentStrategy.hpp

polysolve/nonlinear/line_search/LineSearch.hpp