* Implement more wavelet types:
  Check the literature to find out what are commonly used types. Candidates
  could be coiflets and symmlets.
** Coefficients for coiflets and symmlets found so far are only with a
   precision of about eight digts. This is probaly insufficient.

* Wavelet packet transform:
  Should include utility functions for selecting the coefficients according
  to "dwt-type", "best basis" or "best level".

* Continuous wavelet transform.