Improve array filling/accessing for Operator.__call__
Improve array filling/accessing for Operator.__call__

When possible, replaced ``x[:, :, :] = y`` with  ``x.fill(y)``.
Simple performance tests showed the approaches to be identical
with respect to process time. The latter maintains more
flexibility; whatever the shape of x becomes, no changes
need to be made, so long as x has a ``fill`` method.

Replaced ``rates[i, :, :] = ...`` with ``rates[i]`` per
reviewer comments

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