Add stubs for all known missing ELM operators
Refactored expressions into multiple files (organized according to ELM specification structure).
Further refactoring could be done, but don't want to cause too many merge conflicts, so stopping
here for now.  Tests should also be refactored similarly, when convenient.
Merge pull request #10 from cqframework/refactor_expressions_and_datatypes

Refactor expressions and datatypes