Make self.elements.moving a boolean array to use for numpy array broadcasting

[Boorhin]

At the moment self.elements.moving is an integer array full of 0 and 1. Its purpose is to select operations only for moving elements
I suggest that it becomes a boolean array as such that it can be directly used to modify only the values of moving (or with np.logical_not() "not moving") elements.

For example:

• at the moment a lot of operations are made as such
  elements.property + self.elements.moving*(mathematic operation)
  That implies that all the array is multiplicated by either 0 or 1
• whereas if it was a boolean array it would be:
  elements.property[self.elements.moving] += mathematic operation
  The operation is applied only to the moving elements.
  This optimisation can accelerate the copmutations significantly in particular in models where particles appear and desactivate all the time. There is no need for computing the whole elements array