Crop representative cells plugin

[fefossa]

New plugin/module idea is to have a module that saves one representative cropped cell for each site.

Beth's suggestion is:

• Use SaveCropObjects code to implement this plugin because it already knows how to crop objects;
• Have one representative cell be selected in each site using a function similar to this one;
• Based on the measurements that CellProfiler does in the steps before, ask the user to select which features will be used to choose the representative cell (the main group features, such as Intensity, Texture, and Granularity measurements).