API: make '+' and '-' for Index either do numeric operations of raise TypeError (instead of setops)
see comments at the end of #8184 

so this type of syntax

`index1 + index2`
would be replaced by
`index1.union(index2)`

and 

`index1 - index2`
by
`index1.diff(index2)`

Their is some internal code to fix but not a lot.

This would go a long ways toward simplifying the user API for index ops. I think.
