add functionality for varying n0, n2 slice-by-slice w ex nb

[nickgoldring]

We changed element.py to accept an array of varying n0, n2 values longitudinally along the crystal. n0, n2 array sizes must match number of crystal slices for obvious reasons. Example notebook demonstrating this functionality for multiple crystal slices and multiple pulse slices may be found in the examples/wavefront directory: pulsePropagateMultiSliceCrystal02.ipynb

[bnash]

Additional checks should be added to ensure the n0 and n2 array sizes match the number of crystal slices. Merging.