Add frozenspins like in MuMax3

[ilateur]

Defines spins that should be fixed

I would add this as a Parameter. If it is not assuredZero(), 1 - frozenSpins could be multiplied by the total torque at the very end, before being used by timesolver.

This is not a very high priority, but the same logic can then be applied to the evaluation of the elasticVelocity in order to freeze some displacement. This can be used for fixed boundary conditions.