Several versions of SVD_k(A_k+X) with X sparse

add: SVDUpdateTest.py
update: IterativeSVDPCA.py
versions of the updater:
    - addSparse: Explicitly compute A_k+X
    - addSparseArpack: Implicitly define the function $f: x -> (A_k+X).x$ and
      use it with Arpack (through scipy.sparse.linalg.eigsh)
    - addSparseProjected: Use SVD update defined in (Horst and Hongyuan, 1997)
    - addSparseRSVD: Use Randomized SVD based strategy