Remove unnecessary forward calculations

[mdtanker]

Currently, at each iteration the forward gravity of the entire prism model is calculated twice. Once for determining the Jacobian, and once for updating the residual misfit after the topography correction is applied. These calculations should be able to be restricted to just the prisms which have been altered during the previous iteration, which will hopefully significantly speed up the code.

The previous version of the Jacobian and the forward gravity of the prisms should be saved, and entries only updated if those prisms have been updated.