Use angular displacement as is in roll-angle

parameter, i.e. the user specifies the real angles by which the sample is rolled,
not the oposite ones. This way is the behavior uniform with roll angle as a
normal scalar parameter not an optimization parameter.

src/kernels/templates/roll_template.in

@@ -5,7 +5,7 @@
 %nl
         voxel.x = mad((float) idx, x_region.y, x_region.x);
         voxel.y = mad((float) idy, y_region.y, y_region.x);
-        sin_roll = sincos (mad((float) idz, roll_region.y, roll_region.x), &cos_roll);
+        sin_roll = sincos (mad((float) idz, -roll_region.y, -roll_region.x), &cos_roll);
         tmp = mad(z_region.x, sin_lamino, y_center);
         tmp_x = voxel.x * cos_lamino;
         tmp_y = -voxel.y * cos_lamino;