Emancipate sites from `builder`
Kwant is moving in a direction where sites and the associated families and symmetries are becoming first-class concepts that are independent of the builder module.
As a first step in this direction, let's move Site, SiteFamily and Symmetry from builder into system. (the old names will have to remain usable for backwards compatibility.) SimpleSiteFamily can be also moved and perhaps renamed.
This will allow to make the lattice module independent of builder. This in turn will allow the use of concepts like TranslationalSymmetry inside the builder module (currently this is impossible due to circular imports.)
With site families independent of builders VerySimpleSymmetry can be replaced by TranslationalSymmetry in the builder tests.