moved pruning code to SparseVector.h
I'm planning to start working on the storage code for sparse matrices. There are a few feature requests for this, see below. This MR does not yet address any of those, but instead moves the prune function, which is only used (and useful) for sparse vectors from the storage implementation to the sparse vector implementation. As this is not related to how the data is stored, this shouldn't (imo) be part of the CompressedStorage class anyway.
So this can be seen as a first cleanup step.