time_bucket_gapfill does not support running without datapoints param.

[dreaquil]

Hi!

I'm trying to run a query with time_bucket_gapfill as follows:

MyModel.timescale.filter(time__range=ranges).time_bucket_gapfill('time', '1 day', start=ranges[0], end=ranges[1]).annotate(Count('temperature'))

However, since datapoints defaults to 240, it's not possible to pass None to the interval.

[dreaquil]

Opened a pull request #36. However, it seems to me that datapoints is not working properly. The behaviour intended by the timescaledb documentation does not match the one used here.