Don't allow dots '.' in Identifier

[sapols]

We decided in #156 to temporarily allow dots '.' in Identifier to avoid solving namespacing issues. We need to disallow dots again.

The main sticking point is DataType.flatten where we're prepending parent Tuple names to Scalar IDs. We'll have to come up with a different way to keep track of parent Tuples—perhaps a metadata property that stores the dot-separated names.

Then there are other places like DataType.findAllVariables and DataType.getPath that perform special logic using the dot-separated IDs. We'll have to refactor those places to use whatever new construct we come up with.

There are TODOs in the code reminding us what places need to be changed once Identifier disallows dots again. Check:

• identifier.scala
• IdentifierSpec.scala
• parsers.scala (specifically, def identifier)
• ConstraintParserProps.scala (specifically, val identifier)