Some backend API remarks

[pwl]

Should we change the fallback of output to an error? Although the current state might be convenient for us we don't want to impose (via suggestion) any particular requirements on the internal structure on AbstractState. This is only one simple line for the user to implement. Also do we need order, name and isadaptive now?

[mauro3]

Error for output sounds good.

If not for us, I think we still need to provide means to query the important aspects of solvers, i.e. order and adaptivity. The name function should be useful for printing.

[pwl]

As of #19 output is no longer defined for abstract solvers. We still hve order, name and others do deal with.