Added code to compute volume of closed regions.

The volume of closed objects is needed for concentration
based volume release.
Added exact concentration based volume releases for single regions.

Previously, concentration based volume releases would happen
by picking molecule positions in the closed volume's bounding box
(at the proper concentration) and only molecules that would fall
within the actual region would be placed. However, since the
computation of the number of molecules would happen based on the
volume of the bounding box, the number of released molecules
within the region would fluctuate from seed to seed and within
a given seed for different regions of identical volume. This
behaviour seems counterintuitive. Regions of identical
volume should receive identical numbers of molecules (placed
at random positions) unless such a fluctuation in number is
requested via a (not yet implemented) mdl keyword.

The current commit enables these exact region releases based
on region volume for single closed regions without any boolean
operators. For regions composed via boolean operators the code
still uses the previous bounding box based release. This is due
to the fact that the computation of volume for regions composed
via boolean operators requires more sophisticated CSG methods
currently missing from MCell. This may be addressed in the
future.
Updated gitignore.
Removed some unused debug code.
Removed unused subdivded subvolume code.
Removed unused code related to inserting/excerting volume molecules.
Removed unused function for randomizing volume molecules' positions.
Removed superfluous debugging code and comments.