start_datetime needs to be not null

[mandagill]

at line 21 of data_filter.py, the function uses the start_datetime attribute of a data point object to determine if it needs to call for more data. At this time, that column is nullable in PostgreSQL.

Desired resolution: need to add a check for whether this is null or not; if it's null, assume the user is new and create a new record. Otherwise duplicate DB records are created.