Casting to float64

[chourroutm]

Hi,
In version 2.0b2, the data is being cast as float64, and I recall reading that Neuroglancer does not support float64 data at the moment. See google/neuroglancer#875 and screenshot below:

As it's Neuroglancer-oriented, adding a warning would probably be sufficient

[dstansby]

Snap, I just got the same issue! Fixed by #172