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.

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

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

Conflicts:
	kwant/builder.py

make string representations of named lattices and related objects useful by making them much more compact

allow attaching a lead to a builder that contains sites that are not commensurate with the leads's symmetry