Rename representative_elementary_volume to rev_porosity and create others like rev_tortuosity

[jgostick]

The title is self-descriptive, but I think we need to make room for other definitions of representative elementary volume. I think using rev_<metric> might work.

In addition to rev_tortuosity we could do things like rev_rmax which computes the largest sphere that can be drawn in each subvolume. There are actually many possibilities.