EAGLESIX-mongo_2.6.5_expressions_Expression made another pass through Ex...

This pull touches three files - Expression, ConstantExpression, and FieldPathExpression.  The former includes several changes. The changes are mostly trivial.  However, one of these requires team review.  Changes to ConstantExpression and FieldPathExpression included added a parse function to conform to the c++ code.  The parse method in both cases is trivial.