Improve class documentation

Whenever a paper is mentioned, including the corresponding reference (i.e. full bibliography cite and/or link) would be nice.
See for example, the block
` * \brief Wavelet based on paper VOW: Variance-Optimal Wavelets for Steerable Pyramid (P.Pad et al 2014).
`
in `itkVowIsotropicWavelet.h`.

On the other hand, formulas must follow Doxygen rules [1]. There are a few examples in ITK, such as [2].

[1] http://www.stack.nl/~dimitri/doxygen/manual/formulas.html
[2] https://github.com/InsightSoftwareConsortium/ITK/blob/master/Modules/Filtering/ImageNoise/include/itkShotNoiseImageFilter.h