Refactor SchurComplement operator.

Refactor SchurComplement operator.

Subsequent to the implementation of Payloads for LinearOperators, it is
necessary that compound operations such as this one be expressed in
terms of overloaded operations (operator+,* etc.) instead of in a
low-level format, with the operator functions v_mult etc. expanded
manually. This is because the payload is not correctly configured in the
latter case, and the operations defined on the Payload-level would not
align with those being performed by the higher-level LinearOperator
interface. When operator+,* etc. are called, the equivalent operations
for the payload are automatically set up.