Feature request: Support multiple input vectors for eigsolve

[lkdvos]

Sometimes, it might be useful to have the option to start with multiple initial vectors.
In particular, when the first 2 dominant eigenvectors are known, but one would like to know the first 4, there is currently no method to pass the information of the second eigenvector, and this one will have to be determined from scratch.

I don't know how much this affects the runtime, but it could be a cool feature to add.