Merge branch 'kpm_new' into 'master'

+ 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