Added function for saving execution statistics to spreadsheet.
Added function for saving execution statistics to spreadsheet.

This saves us the trouble of manually extracting these numbers from the
log file. This introduces a dependency on the xlwt package (in PyPI), but
RMG will skip writing the statistics file if it is not found rather than
failing completely.

comocheng/RMG-Py,faribas/RMG-Py,faribas/RMG-Py,nickvandewiele/RMG-Py,comocheng/RMG-Py,enochd/RMG-Py,pierrelb/RMG-Py,enochd/RMG-Py,nickvandewiele/RMG-Py,nyee/RMG-Py,KEHANG/RMG-Py,chatelak/RMG-Py,KEHANG/RMG-Py,nyee/RMG-Py,chatelak/RMG-Py,pierrelb/RMG-Py