Allow pywt.Wavelet to be passed as an argument of dwt functions

I think it would be convenient to be able to use pywt's Wavelet object as argument of dwt() functions, e.g. to have something like:

dwt(x, wavelet=pywt.Wavelet('haar'))

instead of:

dwt(x, hfilter=np.array(pywt.Wavelet('haar').dec_hi), lfilter=np.array(pywt.Wavelet('haar').dec_lo))

(of course we could keep hfilter and lfilter as alternatives parameters)