Created module for data types for tables.
Created module for data types for tables.

* Created class `TextValueType` to represent data types whose values can
  be textual atoms.
* Instantiated data types: atom, bool, int, float, string, date, time,
  datetime.
* Wrote function to parse data type declarations.
* Wrote functions for parsing dates and times.
* Made map of data type names to data types.

afbarnard/fitamord