Cleanup of common rules code.

Rules which multiple parsers can use are placed in splparser.cmdparsers.common. Due to the refactoring from monolithic to decentralized parsers, the splparser.cmdparsers.common rules package was full of unnecessary rules, and it wasn't clear, when creating new parsers, what rules to use. This has been remedied, but required a lot of cleanup work in existing parsers, as well as in common rules. All tests pass. An explanation for developers on how to use the common rules will be on the wiki.