Fixed bug affecting removal of volume molecules.

Issue: Volume molecules would not be removed when using a
release site with negative molecule count and a closed
region as release shape.

This issue only affected molecules for which no reactions
were defined since internally these would be represented
with a simplified data structure (compared to molecules
which have reactions defined). The code for removing molecules
uses this data structure and would thus do nothing for
molecules without reactions. To fix the issue, I made sure
that all molecules have this data structure. This may result
in slightly more overhead for molecules without any reactions
but it doesn't seem likely that this would have any major
impact on code performance.
Abort negative molecule releases via SHAPES and LISTS.

Negative molecule releases (to remove them) are only supported
on or within regions. However, previously the parser would
still allow negative releases on SHAPES with no effect (or
perhaps worse). I've changed libmcell to only allow negative
molecule releases from regions. Anything else is rejected.
Updated copyright date to 2014.