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.
I'd suggest having a generic function in string.F90 called `function zero_padded(number, width)` that returns the number with the requested width, i.e. zero_padded(10, 4) would return the string '0010'.