Improve and make weight_power_scale public
Improve and make weight_power_scale public

Make it more practical for external users (specifically katsdpcal) to do
the weight scaling themselves, by
- making corrprod_to_autocorr and weight_power_scale public functions
  (no leading underscore); and
- allowing the caller to supply the buffers used, to facilitate reduced
  copies and memory allocation (which may be important to performance in
  katsdpcal).

ska-sa/katdal