Make the grid symmetric along the x-axis

This is actually what is claimed in the paper.  Previously, the mirror
axis was y=0.005.  Having the grid symmetric across the x-axis itself
simplifies the expressions for the boundary conditions a little bit.

[[Imported from SVN: r10029]]

cosserat-continuum-twisted-strip.parset

@@ -3,8 +3,8 @@
 #############################################
 
 structuredGrid = true
-lower = 0 0
-upper = 0.1 0.01
+lower = 0  -0.005
+upper = 0.1 0.005
 elements = 10 1
 
 # Number of grid levels

twisted-strip-dirichlet-values.py

@@ -10,18 +10,12 @@ class DirichletValues:
         angle = self.totalAngle * x[0]/self.upper[0]
         angle *= self.homotopyParameter
 
-        # Center of rotation
-        center = [0, 0, 0]
-        center[1] = self.upper[1]/2.0
-
+        # Rotation matrix (around y-axis)
         rotation = [[1,0,0], [0, math.cos(angle), -math.sin(angle)], [0, math.sin(angle), math.cos(angle)]]
 
-        inEmbedded = [x[0]-center[0], x[1]-center[1], 0-center[2]]
-
-        # Matrix-vector product
-        out = [rotation[0][0]*inEmbedded[0]+rotation[0][1]*inEmbedded[1], rotation[1][0]*inEmbedded[0]+rotation[1][1]*inEmbedded[1], rotation[2][0]*inEmbedded[0]+rotation[2][1]*inEmbedded[1]]
+        # Matrix-vector product, vector is [x[0], x[1], 0]
+        out = [rotation[0][0]*x[0]+rotation[0][1]*x[1], rotation[1][0]*x[0]+rotation[1][1]*x[1], rotation[2][0]*x[0]+rotation[2][1]*x[1]]
 
-        out = [out[0]+center[0], out[1]+center[1], out[2]+center[2]]
         return out