Altered:
- how the bounding rect was computed for the color legend.
- Where the legend sits be default.  Now it always seems to paint properly.
Altered:
- Changed MPlotInterval to MPlotRange and changed it from a QPair to a QPointF so I could have a null object to work with.
Added:
- mutable keyword so the range can be cached in subclasses.
Merge branch 'Optimization' of https://github.com/acquaman/MPlot