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.

[mdtanker]

With #118, we now enable remove prisms which are marked with a nan in the mask variable of the model dataset. This enables removing prisms from both the forward calculation and creating the Jacobian. For prisms which don't change much after an iteration, we could add them to the mask to speed up further iterations.