Add localoperator wrapper

This merge request adds several local operator wrappers that take a local operator and assemble (or apply) only parts of the Jacobian, eg the block diagonal or the block off-diagonals.

They will later be used to test the implementation of matrix-free solvers and preconditioners. For this specific task it is more efficient to use special local operators that were written to only assemble the needed parts instead of using these wrappers, but they make testing and prototyping much easier.

fixes #143 (closed)