Add time dependence to formula variables

Textured surfaces are time dependent, but not the formula variables so you can't do time dependent formula calculations.

Also the facet details are not time dependent either. Actually, the facet details don't even calcuate the steady state values of the derrived quanities like pressure and etc; just displays "0". These would be an average over the whole surface so I would labeled it as <p> or mention the fact in the manual that these are only average values over the facet surface. Also, how do you display derrived values for 2-sided surfaces which can have different values on either side? Though this starts going down the path of our much bigger and complicated discussion of how to expand on the types of hits to count in order to properly calculate derrived quantities, so I'll just quit here for this topic.

From the beginning:

- Yes, facet counters can store only a single value (as opposed to textures and profiles), so quantities derived from facet counters (formulas and values in the facet detail window) show the steady-state value. I would have to change most internal variables, the interface and the save file format to make these counters time-dependent, which I plan to do but a little bit later.

- If the outgassing is limited in time (i.e. 0 at the latest moment), then the steady-state value is consequently 0, which is displayed in the facet details window. Are you getting 0 in other cases? Is the "calculate constant flow" checked in the Moments Editor?

- All facet detail window values are averages over the whole facet. (I thought it is obvious: showing anything else as a single value for a facet wouldn't make sense in my opinion).

- For two-sided facets, I always average the two sides. We expect the user to create two opposing 1-sided facets if he wants to differentiate quantities in the two opposing directions.