Solve the displacement problems properly

Also, do a 1-6-0 TNNMG with (six) V-3-3 cycles

src/one-body-sample.parset

@@ -25,19 +25,19 @@ width = 5
 tolerance = 1e-10
 
 [solver.tnnmg]
-maxiterations = 1 # Krause modification; old: 1000000
+maxiterations = 1000000
 fixed_point_iterations = 10000
 fixed_point_tolerance = 1e-10
 
 [solver.tnnmg.linear]
 maxiterations = 1000000
-pre   = 1
+pre   = 3
 cycle = 1  # 1 = V, 2 = W, etc.
-post  = 1
+post  = 3
 
 [solver.tnnmg.main]
 pre   = 1
-multi = 4  # number of multigrid steps
+multi = 5 # number of multigrid steps
 post  = 0
 
 [localsolver]