ENH: Add support for composite ROIs

[scottclowe]

I based this on these sources:

• https://imagej.nih.gov/ij/developer/source/ij/io/RoiDecoder.java.html
• https://imagej.nih.gov/ij/developer/source/ij/gui/ShapeRoi.java.html
• https://docs.oracle.com/javase/6/docs/api/constant-values.html#java.awt.geom.PathIterator

The composite type used to be returned a rectangle containing only the bounding box around the composite ROI. We still return this bounding box data, but now also the paths contained within the composite ROI. The 'paths' field of the composite ROI contains a list of paths describing the ROI. Each path is made up of a series of nodes, encoded as a pair of values in a tuple. We support the ability to decode PathIterator segments which are quadratic or cubic, but it doesn't appear that ImageJ ever saves such paths (it appears to be impossible to save a composite ROIs with sub-pixel resolution and hence paths are always linear).

Closes #9.

[hadim]

Thank you for this! Looks like you need to rebase.

[scottclowe]

Ah, I was expecting this. Will rebase onto master now.