Fix const inconsistency with dt method that caused problems in bindin…

…g code

cxx/include/mspass/algorithms/deconvolution/MTPowerSpectrumEngine.h

@@ -103,7 +103,7 @@ class MTPowerSpectrumEngine {
   length.*/
   int fftsize() const { return nfft; };
   /*! Retrieve the internally cached required data sample interval. */
-  double dt() { return operator_dt; };
+  double dt() const { return operator_dt; };
   /*! \brief Putter equivalent of df.
 
   The computation of the Rayleigh bin size is complicated a bit by the folding