PowerSpace volume factors
As mentioned in the NIFTy paper, there are many different ways to define volume factors for the PowerSpace.
If I remember correctly, the agreement that we reached some time ago was that
PowerSpace.weight() should use a weight of 1, i.e. it should not do anything. However, the current implementation uses
rho as weights.
I suggest to change this to 1 as discussed. The advantage of using this weight over the others is that the user does not need to do any additional corrections when they have to use a different weight; they can just apply their own volume factors without needing to undo anything the space did by default.