Use numpy.float32 for PixelsType 'float'

See: https://github.com/openmicroscopy/openmicroscopy/issues/2547

Fixes numpy type to use with 32-bit 'float' datatype, explicitly specifying float32 type.

To test, import an image of 'float' datatype, and check we get 'float32' when we do:
```
>>> i = conn.getObject("Image", 8502)
>>> p = i.getPrimaryPixels()
>>> plane = p.getPlane()
>>> plane.dtype
dtype('float32')
```