[eigen] Eigen/FFT

[Mark Borgerding <mark@xxxxxxxxxxxxxx>]

Comments welcome.
http://bitbucket.org/mborgerd/eigen2_for_fft

Done:

• complex & real, fwd and inverse FFTs for float,double,long double
  
• kissfft and fftw implementations, selectable as a template parameter
  
• default is kissfft unless fftw3.h is included before Eigen/FFT
  

TODO:

• multi-dimensional transforms
• IMKL backend
• interface functions that use Eigen matrix types