Clean up traits interface.

[jaidevd]

Most of the traits interface needs to be heavily inspected and if necessary, rewritten. Especially need to examine the need for custom traits, now that excepts in traits are understood.

• [x] Remove all meta/shadow/custom traits that are not necessary
• [ ] Parser arguments for pandas should not be writable, only the schema should be writable. Re-evaluate the API around setting specs throughout PySemantic
• [ ] For SchemaValidator._get_parser_args, use a mapping from schema keywords to pandas parsers keywords.