Minor refactoring.
Focal point calculators are now specific to a source.
Removed calculate_focal_points method form Singlet calculator.
TripletFocalPointCalculator now takes a reading and a frame_index and
returns an x,y tuple.  The FocalPoint class has been removed.
Renamed FocalPointCalculator to FocusCalculator.
Sketched out basics for downloading triplets for a source and workunit.
Introduced CutoutGrid class which the DisplayableTriplet takes as input.