Added zero-padded statepoint names
Wrapped batch number zero padding into a function
Added zero padded filenames for source files

Also correcteted statepoint filenames in write_source_point
Updated tests for zero padded filenames
Commented zero_padded_batch
Merge branch 'develop' into zero_padded_statepoint
Moved zero-padding features to string.F90
Fix distance-to-surface calculationg for general plane surface.
account for differences in band probabilities at different energies
Fixed zero_padded error message typo
Fixed overflow error in count_digits
updated test results
Have update_results.py run OpenMC twice when generating results for
test_source_file. Closes #288 on github.
Merge remote-tracking branch 'mit-crpg/openmc/develop' into band-samp
update source file test reference results
Merge pull request #299 from paulromano/plane-distance-fix

Fix distance-to-surface calculation for general plane surface
threadsafe particle restart writing
Merge pull request #300 from walshjon/band-samp

account for differences in band probabilities at different energies
Allow zero atom/weight percents in materials. Closes #301 on github.
Don't use PWD environment variable when setting path to input files. Closes #302
on github.