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

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

It was not previously made explicit anywhere in the docs on how
the leads were numbered. This commit adds a line to the docstring
of `kwant.builder.Builder.attach_lead` which explains the meaning
of the return value. It also adds a few lines to the first tutorial
explaining the numbering.

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