inplace update of jacobians

[martinjrobins]

the NonLinearOp and LinearOp have a jacobian function which returns a matrix

fn jacobian(&self, x: &Self::V, t: Self::T) -> Self::M

This requires creating a new matrix everytime the jacobian is recalculated. Need an "inplace" version that can update an existing jacobian.

This will require additions to the Matrix trait to allow for mutation of existing matrices

related to #6 and #7