Allowed nu=True in DiffusionCoefficient
Allowed nu=True in DiffusionCoefficient

Added nu-diffusion-coefficient to mgxs_types.

Relying on TransportXS to call the nu-dependent tallies
and assuming those are being used in DiffusionCoefficient
when called with nu=True.

shikhar413/openmc,walshjon/openmc,paulromano/openmc,shikhar413/openmc,paulromano/openmc,walshjon/openmc,mit-crpg/openmc,shikhar413/openmc,mit-crpg/openmc,amandalund/openmc,mit-crpg/openmc,amandalund/openmc,amandalund/openmc,walshjon/openmc,amandalund/openmc,paulromano/openmc,shikhar413/openmc,paulromano/openmc,mit-crpg/openmc,walshjon/openmc