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.