maint: clear up warnings in tests

See merge request kwant/kwant!270

This silences a deprecation warning on scipy < 1.2 and numpy > 1.15.

add automatic Peierls phase calculation

Closes #19

See merge request kwant/kwant!219

Co-authored-by: Pablo Piskunow <pablo.perez.piskunow@gmail.com>
Co-authored-by: Daniel Varjas <dvarjas@gmail.com>

use the shift-inverse method for finding sparse eigenvalues
in the finite system tutorial.

See merge request kwant/kwant!265

This avoids "sparse efficiency" warnings.

add testing environment specification as Dockerfiles and conda environments

See merge request kwant/kwant!260

Sometimes the testing jobs fail because the build artifacts
are removed.

Only builds testing images when the relevant dockerfile/conda envs
have changed.

+ factor kernel (jackson or lorentz etc.) into separate functions, and
  expose these as part of the API
+ factor out the random vector factory, and allow to specify finite
  collections of vectors (e.g. for sampling over some subdomain)
+ implement KPM expansion of correlators, and add a factory function
  for calculating conductivity.

Closes #138

See merge request kwant/kwant!218

allow computations even if a discrete symmetry is broken

Closes #236 and #242

See merge request kwant/kwant!257

Previously if there were more site families than colours in the
matplotlib cycle, then plotting would raise an exception. Now
we cycle through the colours indefinitely.

Closes #257

This was fixed on stable, but due to interim changes and a badly
solved merge conflict, the fix was not applied to master.

This has a lower chance to give an impression that the substitution
happens in-place.

Closes gitlab issue #237

The 'mpl_available' flag was relegated to '_plotter' in
the development version, but the check in the test was still
referring to 'plotter'. The change 'plotter → _plotter' was
made on the development version, so this fix does not need to
be backported to the stable branch.