FEAT: add scipy.sparse compatibility

The class scipy.sparse.linalg.LinearOperator defines an interface that allows the use of several solvers for sparse linear systems. Implementing this interface would give access to these tools for all talon linear operator.

This commit modifies the talon.core.AbstractLinearOperator class to implement the scipy.sparse.linalg.LinearOperator interface. All talon linear operators are now scipy compatible.

Closes issue #36 (closed).