Better documentation of the relative depth factor

The comments were missing the 1/H fraction earlier. This is shortened
away in (most of) the calculations, so that's probably why it was left
out.
Correct sign in finding mobile CO2 interface

Also clean up the code to be more readable/debuggable by splitting up
and giving name to each of the intermediate calculations.
Elaborate documentation for depth-averaging function
Add comment about the equation for upscaled poro
Require that saturation has been updated before downscaling

This has always been a requirement; if the code didn't do it, it was
done automatically. However, the routine is only called internally,
so we have full control over the call site and can set the order of
events ourselves.
Eliminate the check_res_sat routine

There was no longer any use for this and it is now integrated into
the upd_res_sat which is called externally.
Be const-correct to allow further optimization
Recalculate residual interface for any saturation

The rel.perm. function may be called with a saturation which is not
current, and we need to account for what the residual interface would
be in that situation.
Always take into account that we may have a new max. sat.

The saturation we have been asked to calculate with may be the newest
maximum, but it hasn't been set as the official one yet.
Eliminate storage of the residual interface

We *do* store the maximum official saturation that has been seen and
use that if it really is the largest, but we now also recalculate the
interface if we're asked about a saturation which is even higher.
Merge pull request #44 from rolk/044_res

Fix use of historical saturation when evaluating psuedo-func.