Big rewrite in the Sparse module: SparseMatrixBase no longer inherits MatrixBase.
That means a lot of features which were available for sparse matrices via the dense (and super slow) implemention are no longer available.

All features which make sense for sparse matrices (aka can be implemented efficiently) will be implemented soon, but don't expect to see an API as rich as for the dense path.

Other changes:
* no block(), row(), col() anymore.
* instead use .innerVector() to get a col or row vector of a matrix.
* .segment(), start(), end() will be back soon, not sure for block()
* faster cwise product