improve lll and voronoi algorithms in kwant.linalg
Improve the logic of 'cvp', now it provably works in all cases. Add 'reduced' option to 'linalg.lll.voronoi', this eliminates superfluous Voronoi vectors that do not have a corresponding face of the Voronoi cell, or this face would be smaller than 'rtol'. Simplify 'lattice.Polyatomic.neighbors' by providing the option to group equidistant vectors 'group_by_length' in 'linalg.lll.cvp'. See !165 for details. Closes #163.