Add rescaling workflow

This merge request adds the long-requested rescaling workflow. There is a new Subframe, SubframeRescale at the end of the workflow (post-merge), in which the user can currently merge a rescale a single peak collection/data set.

The resaler module does non-linear constrained optimisation of per-image scale factors using the NLopt library. There is one scale factor per image, initialised to 1; the first image is constrained to 1 and subsequent images have their scale factor to be within 5% of the previous image's scale factor. The objective function computes the Rmerge of the merged peak collection, and minimises it by allowing the scale factors to vary.

The widget currently plots the scale factor as a function of image number and saves the scale factors to the log file.

Resolves #679 (closed), #661 (closed)