NewtonsDampedLaw should be parallelized

[Václav Šmilauer]

With the advance of sweeping collisions, NewtonsDampedLaw takes considerable time (over 20%) of simulation. For very large number of bodies, it can be beneficial to run it in parallel.

A prequisite is to handle clumps members on-demand rather than in the main loop (where they should be just skipped), since if running in parallel, it cannot be guaranteed that the clump itself will be processed later even if its ID is higher.

[Václav Šmilauer (eudoxos)]

Clumps memebrs are handled on-demand (when handling the clump itself) now in NewtonsDampedLaw::handleClumpMember (r1853)

[Václav Šmilauer (eudoxos)]

Once per-thread maxima are implemented in VelocityBins, the loop in NewtonsDampedLaw can be parallelized as-is. The rest is hidden behind calls to VelocityBins::binVelSqInitialize, binVelSqUse and binVelSqFinalize.

[Václav Šmilauer (eudoxos)]

Implemented and tested in r1863. Scales quite well.