Hide the repeated mod-div arithmetic

From @ambrad:
We could imagine at some point as a design issue making a bunch of Homme::parallel_for's that hide this somewhat frequently repeated identical div-mod arithmetic and provide the functor with (ie,iq,igp,jgp,k) and variants.

Start by locating the existing Homme::parallel_for call sites and the repeated div-mod arithmetic described in the issue. Compare the variants that provide (ie, iq, igp, jgp, k), then clarify the intended shared interface; done means the design is agreed and the repeated arithmetic is hidden behind the parallel_for helpers.

Written by the indexing model from the issue text.