ts/wf/cross [FEATURE]: let cross oprionally operate in the Fourier domain

For large time series operation in the Fourier domain is computationally more
efficient than discrete time domain convolution or cross-correlation.

src/ts/wf/cross.cc

@@ -4,7 +4,7 @@
  * ----------------------------------------------------------------------------
  * 
  * \author Thomas Forbriger
- * \date 16/11/2016
+ * \date 22/11/2016
  * 
  * cross correlation
  * 
@@ -32,13 +32,13 @@
  *  - 16/11/2016   V1.2   provide new features:
  *                        - full libdatrwxx support
  *                        - trace selectors
- *  - 21/11/2016   V1.3   operate in the Fourier domain to speed up
+ *  - 22/11/2016   V1.3   operate in the Fourier domain to speed up
  *                        computation for large time series
  * 
  * ============================================================================
  */
 #define CROSS_VERSION \
-  "CROSS   V1.3X   cross correlation and convolution"
+  "CROSS   V1.3   cross correlation and convolution"
 
 #include <fstream>
 #include <iostream>
@@ -256,17 +256,21 @@ FourierProcessor::Tspectrum
 {
   TFXX_assert(sref.size() == ssig.size(),
               "inconsistent sets of coefficients");
+  // imaginary unit time pi (shift factor)
+  const std::complex<double> expfac(0.,M_PI);
   FourierProcessor::Tspectrum result(sref.size());
   result=FourierProcessor::Tspectrum::Tvalue(0.);
   aff::Browser<FourierProcessor::Tspectrum::Tcoc> BR(sref);
   aff::Browser<FourierProcessor::Tspectrum::Tcoc> BD(ssig);
   aff::Iterator<FourierProcessor::Tspectrum> IR(result);
+  int l=0;
   while (BR.valid() && BD.valid() && IR.valid())
   {
-    *IR = (*BR) * conj(*BD);
+    *IR = conj(*BR) * (*BD) * std::exp(double(l)*expfac);
     ++IR;
     ++BR;
     ++BD;
+    ++l;
   }
   return(result);
 } // FourierProcessor::Tspectrum