Clean up src/express, src/exppp

* a3186c1 adds a warning for extremely small REALs in the schema that may be misinterpreted by other software if stored in a `float`

* cd530f1 causes all warnings and errors from the parser to print with a unique identifier `PEnnn` or `PWnnn` to make it easier to filter out the messages of interest.

* gcc prints **no warnings** for src/express and src/exppp