Add Time dimension to TileDB schema

[kylemann16]

Add Time dimension to the TileDB schema. Because of the way we need to use the time traveling abilities of TileDB, we need to group Shatter tasks together in those time slots. This means we need another way to track time. Currently we're using meta data to track that, but it would probably be better to utilize a 3rd dimension for TileDB and add Time there.

This way we can query cells by date, and since we're already requiring that users have a date associated with the Shatter command, we can use that date to ingest the data into TileDB.

[kylemann16]

Addressed in #123.

Landed on adding start_datetime and end_datetime as attributes in a similar way to count. This way, users can query the database by start and end datetimes, and we don't remove the ability to manage the database using tiledb time traveling

Other things I tried:

• adding a time dimension to the schema, but this ended up creating too much extra data
• repurposing the tiledb timestamp when writing to do dates for shatter processes, but this ended up being too invasive for other SM processes, namely extraction. This would add the ability to time travel in terms of actual process dates, but would remove the ability to delete processes by their insertion order, which I think is still very important