make 'monomials' preserve order of operators,
remove some unimportant (redundant) tests,
and improve the API of 'monomials', which is
not yet part of the public API.

Closes #135

See merge request !151

Current behaviour assumed that all generators shall be commutative.
By preserving order of generators we will preserve at least some
information about order they were present in initial formula.
This information is useful e.g. in Landau Levels project. closes #135

"monomials" are not yet a public function. Interface improvment consists of:

- change "*gens" into single argument "gens" with default value "None".
- If left as "None" it reads all symbols present in the given "expression".
- It is also possible to provide "gens" as sequence of strings (symbol names).

Bump required Python version and library dependencies to the
oldest supported ones. With the release of Debian Stretch
the oldest supported platform is Ubuntu 16.04, which has:

+ Python 3.5
+ numpy 0.11.0
+ scipy 0.17.0
+ matplotlib 1.5.1

We are making these changes now in the expectation that we
will not make a new release before 6 months have elapsed, in
line with our policy to support old platforms for 6 months
after they are supplanted.

Closes #31

See merge request !146

Add check for this in setup.py to avoid potentially confusing
tracebacks later.

Remove fixes for unsupported versions of numpy, scipy and matplotlib.

Previously the test suite generated warnings that were swallowed
when pytest was invoked from the command line. Here we enable
printing of warnings generated during testing, and we also
remove the sources of some of those warnings (e.g. using
deprecated APIs).

Closes #131

See merge request !143

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.