(1) Fixed a bug in the BooleanTranslator constructor to only include …

…nodes in the RESET rule that have update functions (some nodes are just used as inputs, e.g., nodes representing inhibitors). (2) Fixed a bug in the ROA mode where null events were not being properly handled. Previously, they were being skipped (which is correct for the GSP mode). In the ROA mode, they need to be retained because the 'reset' mode changes state from 'N' to 'Y'. (3) Made a bunch of small formatting changes to comply with the PEP standard.