Tests for PLUS impact algorithm

Work in progress; PR for discussion only.

<b>Status</b>
Position projection algorithm can be run in <i>exhaustive search</i> and <i>successive pruning</i> modes; impact handler currently performs exhaustive search only. Handlers no longer create internal copies of the MultibodySystem. Results seem reasonable for the six tests run here, though more testing is necessary. Toggle the <i>PRINT_</i> flags to see detailed output.

<b>To do</b>
<ul><li>Implement <i>successive pruning</i> mode for impact handler.</li>
<li>Test in more scenarios and with different parameters.</li>
<li>Compare results with compliant contact model.</li>
<li>Generalize for arbitrary multibody systems.</li>
</ul>