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 would prefer not to use this algorithm.  It will work fine for this specific case of statepoint numbers, but it is not reliable in general.  The inclusion of 'real' causes precision errors.  For example,
```int(log10(real(999999999))) + 1```
will count 10 digits, but the correct answer is 9.