Optimisations to weight power scaling
Optimisations to weight power scaling

- Reduce the dtypes of the index arrays (makes a few % improvement).
- Use np.reciprocal to compute 1/x, instead of the previous method which
  created a temporary array.
- Multiply the weight scale factor with the weight as part of the
  operation, instead of as a separate step (big improvement).

Together these give about a 25% improvement in throughput on my laptop
(when reading from cache). It will probably be less in real-world usage
as it will be more I/O-bound.

ska-sa/katdal