Add parse_data method

[rockdoc]

Add a parse_data() method to the CDLParser base class so that client code can pass in CDL input text as a plain string. The existing parse_file() method can then simply call the proposed new method having read in the text from the CDL file.

It would be nice to support general-purpose stream input, but I don't think the underlying PLY parser supports that approach - need to check this.