[BUG] Incorrect division in k_mask_grid_search

[sizmailov]

R-factor of (f_obs/k, f_model) is not equivalent to R-factor of (f_obs, f_model*k) (where k is an array with non-equal elements). Therefore I believe the next snippet contains an error:

cctbx_project/mmtbx/bulk_solvent/scaler.py

Lines 378 to 383 in 56dfcc6

	bulk_solvent.k_mask_and_k_overall_grid_search(
	f_obs.data()/k_total_,
	core.f_calc.data(),
	core.f_mask().data(),
	k_mask_trial_range,
	selection_use)

Corrected code should be something like:

        bulk_solvent.k_mask_and_k_overall_grid_search(
          f_obs.data(),
          core.f_calc.data() * k_total_,
          core.f_mask().data() * k_total_,
          k_mask_trial_range,
          selection_use)

@pafonine @olegsobolev