Added ability to continue checkpointed simulations.

Previously, simulations that were checkpointed via CHECKPOINT_ITERATIONS
would stop after the specified number of checkpoint iterations
and had to be restarted manually. In contrast, simulations that
were checkpointed via CHECKPOINT_REALTIME could continue via the
NOEXIT keyword. This commit adds the analogous capability to
CHECKPOINT_ITERATIONS via

CHECKPOINT_ITERATIONS = <integer> <EXIT|NOEXIT>

with EXIT being the default for backward compatibility.

In addition, I added the keyword

KEEP_CHECKPOINT_FILES = <TRUE|FALSE>

When enabled this keyword triggers MCell keeping backups of all checkpoint
files generated along the way by tagging files with their respective
iteration number. The default for this option is FALSE, in which case
subsequent checkpoints will overwrite previous checkpoint files
consistent with the current behavior.
Merge branch 'master' of ssh://github.com:haskelladdict/mcellteam/mcell