Fix bug in CommonScaler. Fixes #282
Fix bug in CommonScaler. Fixes #282

If resolutionizer doesn't complete, then all merger.get_resolution_*
return None, making resolution_limits a list of Nones. This change
therefore stops estimate_resolution_limit from returning None, and
will instead return 0.0 as expected.

xia2/xia2,xia2/xia2