Key access for Named Attributes on a BlenderObject

[BradyAJohnston]

Better aligns named attribute access with numpy array access.

bob['position'] # retrieve the position

bob['position'][0] = 3 # set position for first point to (3, 3, 3)

bob['position'][0][0] = 3 # set the x value for first point to 3

bob['new_attibute'] = np.zeros(len(bob), dtype=int) # creates a new attribute, only supports the point domain

Fixes #50