Zero padded statepoint

This is a small PR that will make statepoint file names more user-friendly.  This flavor of OpenMC outputs statepoints and source files with zero-padded batch numbers.  For example, it would output "statepoint.05.binary" instead of "statepoint.5.binary".  The zero-padding has a few convenient advantages such as the "ls" command displaying the statepoints in proper order.
Thanks for the heads-up on the overflow error for n_digits = 10.  I think it's fixed now.