Update OSQP workspace, instead of recreating

[rjoomen]

This recreates the OSQP workspace, instead of recreating it, if the sparsity of the P and A matrices has not changed.

Below are runtime results of some examples (average of three runs each). Warm start is switched on or off using trajopt_solver_profile->settings.warm_start.

	puzzle_piece_example	puzzle_piece_auxillary_axes_example
Cleanup/setup	10.09463	28.40169
Update, warm start	7.164561	24.4617
Update, cold start	7.944992	24.38523

@Levi-Armstrong can you take a good look at this? The examples do show increased performance, but we'll have to be sure there are no side effects. In my own work I did not see any improvements, but my optimization times are much lower.