changed default value to switch to python vs tensorflow based iteration

[timsainb]

This just changes the default switch from iterating with a tensorflow backend vs a python backend at a smaller dataset size.

I was experimenting around a bit and found issues with datasets > 1GB with the tensorflow based iteration. So it seems safer to make the default the python-based iteration/sampling for any dataset bigger than 512MB. The speed gains for the tensorflow-based iteration aren't too big anyway.

[coveralls]

Coverage remained the same at 87.697% when pulling 5fb62c8 on timsainb:tf_gather_indices_in_python into de88b39 on lmcinnes:master.