Rework Subscript ROIs

[mpourmpoulis]

• Rename subscript key to index, in order to free up key for dictionaries

• Clarify behavior for empty upper/lower/step

• add left,middle,right for picking up from chains Like data[x][y][z] and as well as something to pick up the "pure subscript body

• handle better edge cases with extra dimensions. For instance, when you have multiple slices and indices mixed together