Conflicts:
	kwant/physics/leads.py

Joseph Weston authored 9 years ago and Christoph Groth committed 9 years ago

Before this commit, if there was a lead for which
the inter-cell hopping matrix was the zero matrix, then
kwant.physics.modes() returned arrays with uninitialized memory.

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.

Conflicts:
	README

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