Prescribed accelerations should be known by Velocity stage

[sherm1]

Currently they are deferred until Dynamics stage since they are not needed until calculating accelerations. However, some (not-commonly-used) operators allow to compute accelerations from forces given as arguments. Those operators currently require realization through Stage::Dynamics meaning that built-in force elements get evaluated even if they aren't going to be used.

It would be better just to calculate prescribed accelerations in the Velocity constraint cache; they aren't usually expensive.