Add cupy's eigenvalue computations in eigs

[mrava87]

Currently the eigs method of the LinearOperator class differentiates between the CPU and GPU scenarios, where the former uses scipy routines and the latter our own implementation of a power method.

Since cupy has recently added similar routines to those of scipy, we want to add them into the backend module and have a transparent use of the same routines as done in most other cases in the library