provide support for date functions

[tyers]

provide the ability to do date transformations within the sparkml to pmml conversion.
Built in functions which may provide options here:
datediff: provides the abilty to calculate the number of days between two dates. Would likely need to pmml magic to make this work with the pmml "period-since" date functions
to_unix_timestamp: effectively provides direct access to "seconds-since-year" on the pmml side, hard coded to a constant year of 1970.