notebook to verify multi-slice crystal behavior

[nickgoldring]

Notebook verifying multi-slice crystal behavior with multiple propagators can be found here: https://github.com/radiasoft/rslaser/blob/119-verify-correct-behavior-with-n-crystal-slices/examples/wavefront/pulsePropagateMultiSliceCrystal01.ipynb

[bnash]

With multiple slices, it's even more important to get the scale length correct for the LCT propagator. With the right value, agreement is seen between the LCT and SRW based propagators for multiple slices. It remains to create a more general propagator that accepts an array of n0 and n2 values. We tested it on using the same n0 and n2 through the crystal.