consider computing geometry on demand via inlined functions

[zingale]

Right now we carry around 3 area MFs + 1 volume MF. These are not used very widely, and it seems that with the C++ conversion, we could create an inlined function area() and volume() to compute these as needed and apply them immediately. This will free up 4 Real's per zone of memory in MFs.

[maxpkatz]

#734 was our attempt at this in Fortran