slight change to test
trying to synch magnetic.cc with gepore.f
merging
Optimize slab building for large microslab layers

No need to do all that work when blending is a null op
Slab building optimization is now pure numpy

18x speedup over unoptimized case, 1.7x speedup over the earlier
optimization. (When profiling using examples/polymer/tethered.py with
step_interfaces=True)