Kirkland-parametrized ptychography example

Now that examples/lightning.py can reconstruct a reasonable potential from Mark's data, we should try constraining to atomic potentials using the Kirkland parametrization.

Plan

To do this, we should add a flag like --kirkland which when enabled, adds two coarse-scale image parameters: species and position which is relative to the coarse voxel center.

We will need to be able to specify the grid spacing for the parametrization independently of the potential (finer) grid, and we need to be able to specify the atomic species as a list of element symbols. In the training_step we will convert these to a full scale potential then call our existing Multislice object.

Note that for thicker potential slices, it may make more sense to implement 3D FDES using projected potentials. This can be done but should be user-selected, with a warning when using above around .1 A dz.