remove superfluous lattice constant from spin_potential_shape tutorial

The lattice constant is never changed from 1, and in addition can be
varied independently of the 't' and 'α' parameters; This should not
be the case if the model is meant to be derived from a discretization
of a continuum model. We remove the lattice constant to limit confusion.

doc/source/code/figure/spin_orbit.py.diff

@@ -34,10 +34,10 @@
  #HIDDEN_END_hwbt
  
  
- def make_system(a=1, t=1.0, alpha=0.5, e_z=0.08, W=10, L=30):
+ def make_system(t=1.0, alpha=0.5, e_z=0.08, W=10, L=30):
      # Start with an empty tight-binding system and a single square lattice.
      # `a` is the lattice constant (by default set to 1 for simplicity).
-     lat = kwant.lattice.square(a)
+     lat = kwant.lattice.square()
  
      syst = kwant.Builder()
  
@@ -54,7 +54,7 @@
  #HIDDEN_END_uxrm
  
      #### Define the left lead. ####
-     lead = kwant.Builder(kwant.TranslationalSymmetry((-a, 0)))
+     lead = kwant.Builder(kwant.TranslationalSymmetry((-1, 0)))
  
  #HIDDEN_BEGIN_yliu
      lead[(lat(0, j) for j in range(W))] = 4 * t * sigma_0 + e_z * sigma_z