Estimate probe positions

We should be able to estimate probe positions with gradient descent. This is slightly challenging since it requires data to be estimated associated to each probe location, meaning the dataset classes may need updating.

Plan

STEMDataset should be updated to return the probe index in addition to the position and CBED. That index will let us initialize an Nx2 parameter and index into it to find a correction term for the probe position. This correction term can be initialized to all zeros, and regularized with L2 regularization.

Related: #15 (closed) #14