The lead does not have connected unit cells and serves no purpose.
We also remove the check for the number of 'collections' in the
resulting plots, as now both 'syst_2d' and 'syst_3d' produce
systems with 2 leads.

This is the only sane behaviour, otherwise we don't know if our
build is misconfigured because 'build.conf' was silently used instead.

Fixes #133.

  When passing `energy_resolution`, the number of moments is set using
  the yet undefined parameter `self._a`. Now the number of moments is
  set after defining `self._a`.

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.