New offset options, better error messages, new Text node type

1. Better error handling for Param missing kwarg specifying what node it's a parameter of.
2. Add new Text subclass of Node for conveniently putting text into PGMs.
3. Add above and below relative offset options and add kwargs to PGM builder class to allow specification of vertical and horizontal offset defaults.
4. Add extra spacing for plates only if there are plates.
5. Add a copy method for PGM builder.
6. Add state space model example.