Some updates to SVD impl

@vrilleup I made the following changes to the SVD impl:

* use BDV directly in symmetricEigs
* change the computation mode to local-svd, local-eigs, and dist-eigs
* update tests and docs
* simplify axpy

Calling the modes sparse or dense is confusing because all methods applies to both dense and sparse matrices. The differences are whether to explicitly compute A^T A and whether to use LAPACK or ARPACK. So I changed them to "local-svd", "local-eigs", and "dist-eigs". The tests were updated accordingly.

Please let me know your feedback. Thanks!