This undoes 00b0536c, reaching the aim of clearer error messages in a more
systematic way.

Conflicts:
	README

Duck typing generally works well, but it can lead to very confusing error messages.

GreenFunction already has a (private) method `_a_ttdagger_a_inv`
to calculate tt^+, there was code duplication in the implementation
of `_transmission`. Replace duplication by a call to
`_a_ttdagger_a_inv`.

Conflicts:
	kwant/plotter.py