Add a commandline variable to extend the runtime of the simulation

[slarson]

The ideal usage of this would be as an argument to run.sh once #253 is addressed.

Whatever number you have as an argument will be set as an environment variable and incorporated as the simulation time, overriding the default and allowing a user to run the simulation for longer to get more interesting output.

Invalid arguments should return an error message.

[slarson]

I have a fix for this at https://github.com/slarson/OpenWorm/tree/development