Convert timestamp columns to timestamps in DatasetBuilder

[mdekstrand]

DatasetBuilder.add_interactions (and from_interactions_df) should have better handling for timestamp columns, since those are recognized specially by LensKit:

• Convert timestamp columns to Timestamp types.
• Provide an argument to control timestamp units.
• Guess timestamp units from numeric timestamps when units are not specified, aiming to put timestamps in the 1900-2000s.