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

Anton Akhmerov authored 10 years ago and Christoph Groth committed 10 years ago

Conflicts:
	kwant/builder.py

Thanks to Adriaan Vuik for reporting.

Thanks to Bernard van Heck for reporting.

Thanks to Bas Nijholt for reporting this.