adding the historical reference code gepore.f, as published in Chatterji (Neutron Scattering in Magnetic Materials, 2006)
support magnetic incident and substrate media
empty string list in extra compile args breaks compiler
Use is/is not None to check for None; numpy is changing == None to a per-element test
Merge branch 'master' of github.com:reflectometry/refl1d
New sld method

Adding the slabs argument allows universal application of an sld method
to any mixture of materials. Caching the result avoids duplicate
computation since the profile function is expensive.

This modification was pervasive. Is there a way to get Pz, Pw globally
without calling slabs.microslabs(thickness)?