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