Make the 'nx' <--> 'kx' transform about twice as fast.

Previously the 'nx' <--> 'kx' transform was done via a complex matrix multiplication.  Instead this transform can be written as a local operation in 'nx' space followed by multiplication by a real matrix.

To distinguish whether or not this local multiplication has been performed, the Hermite-Gauss basis has gained another basis: 'nx_twiddle'.  This basis is for internal use only, and nx_twiddle is just:
psi(nx_twiddle) = psi(nx) * (-i)^(nx_twiddle)

The previous transform still lives on in XMDS, but because TransformMultiplexer should never choose to use it because its cost estimate is higher than the cost estimate of doing the transform via the 'nx_twiddle' space.

git-svn-id: svn+ssh://svn.code.sf.net/p/xmds/code/trunk/xpdeint@2839 41b19578-b77a-4e3b-9682-c8570d237623