Support Cartesian indexing

[rkurchin]

In lattice-based systems such as those @Gregstrq works on, it will eventually be desirable to support Cartesian and not just linear indexing into AbstractSystem objects.

[cortner]

Very interesting option, but an implementation detail and easy enough to provide. In the interface update for 0.4.x we can just allow arbitrary indices rather than only Integer (needs a small rewrite of the document).