__getitem__ for GridPlot, index the subplots using tuple of int or str by giving subplots names. Subplot has name attribute. adding functionality to gridplot to name subplots in gridplot #27

gridplot[0, 0] # returns Subplot at [0, 0]
gridplot[0, 0].name = "bah"
gridplot["bah"] # returns Subplot at 0, 0

each Graphic can also be given a name attribute for referencing it. Subplot.__getitem__ can take str to reference the Graphic items.

implement a way to get Graphic instances from a subplot.

We can also create a useful __repr__ for Gridplot, Subplot and Graphic.