Fix the gradient of the bending energy.

There were two issues fixed by this patch:

a) The bending energy only involves the third director.
   One must look at it as a function in S^2, not as
   a subset of coefficients of a function to SO(3).
   This means that we have to use a different projection
   in one place.
b) A certain subset of the coefficients (see the code)
   must always be zero, because they are parts of
   3-vectors which are known to be perpendicular to (0,0,1).
   For some strange reason they aren't.  I now hardwire
   them to be 0 anyway.  This makes my test pass.
   Maybe one day I'll find the true reason :-)

[[Imported from SVN: r8626]]