Automatically lower PARTIALITY_CUTOFF to improve completeness

[biochem-fan]

When the completeness is very low (say < 90%) after merging, we should automatically lower PARTIALITY_CUTOFF to 'fill-in' missing reflections.

[helenginn]

This will be addressed when sigmas can be calculated more effectively for single-observation reflections and so the sigmas can be appropriately inflated for low partiality reflections.
This will therefore rely on processing being saved to HDF5 file format.