lowercase FAQ.rst, FAQ.py, etc., give meaningful names to labels,
and stop building an user-visible tutorial/faq.py

Apply pep8 fixes and refactor some sections to be more idiomatic
(e.g. avoiding triple nested loops). Also some minor reorganisation
of the sections to give the text a better flow.

That test fails because the result of the selfenergy callback is used as a
numpy array without a call to asarray.

Calling "expand" at this point is not required and differs from
sympy behaviour without a good argument for it. Closes #146.

Older pytest does not have 'warns'.

Closes #132

 The total density of states now integrates to `N`, the size of the system.

This option is not necessary since a similar effect can be achieved by
filling an emtpy builder and using Builder.update.  Furthermore,
removing it opens the door for making fill "sticky" in a consistent way.

Our baseline is Debian stable (Jessie) where possible.  The only
dependency that cannot be satisfied is sympy; Debian stable does not
have a Python 3 package for this.  We set the required sympy version to
that present in Ubuntu 16.04 LTS.

In addition to basic updates to skip permanent redirect, remove an
obsolete reference to macports installation instructions on the main
website.

  `integrate` is a more accurate name for a function that integrates
   the spectral density. Docstrings and tutorial updated.

If "expr" is already a SymPy object and "locals" are provided the
RuntimeWarning will be triggered.