Specifying this is presently optional, in order to remain
backwards compatibility, but will be compulsory in Kwant2.

The inverse map is created during finalization anyway, now
we just save it along with the `sites` map.

fixes #36

this fixes #35

Christoph Groth authored 9 years ago and Anton Akhmerov committed 9 years ago

Co-authored by Anton Akhmerov <anton.akhmerov@gmail.com>

I try to cleanly separate between information that should be part of the
Kwant source distribution (that is tied to a particular version of
Kwant), and generic information that is best presented on the website.