Adapted a mail from Mark about some design and add it as documentation for the FFT module.
Adapted a mail from Mark about some design and add it as documentation for
the FFT module.

pthulhu/eigen,pthulhu/eigen,pthulhu/eigen,cjntaylor/eigen,cjntaylor/eigen,pthulhu/eigen,cjntaylor/eigen,cjntaylor/eigen,pthulhu/eigen