Make default test set repr a pretty print

- change %show magic to only ever display the current selection
__repr__ must return a str or ipython gets a belly ache
Nearly complete the callspec filtering feature

- still and outstanding bug with slices such as ts1.<csid>[:2]
  specified in the getattr/tabbed path
- bail with err msg in _tprint if no items are provided
replace more illegal chars, think about exc re-raising, shit is still badly broken