DateTimeValue and some attr/value reorganisation

[ialarmedalien]

• adds a new type of attribute-value for representing date times
• reorganises the schema a bit to make more use of slots
• removes a redundant unit type
• removes the class NamedQuantityValue, which was used for some attributes that have now been removed (depth, altitude, elevation).

If this is approved, I will update the sample data to match this format.

See also #62

[jeff-cohere]

This looks good to me!