Adds Gaussian type filters with general power exponent.

Add filters of the form

\hat{G}(k) = \exp\left(-\frac12 \sigma^n k^n\right).

This allows to have a sharper filter cut-off without creating Gibbs oscillations. For now, I just added "Gauss4" and "Gauss6" as options. The filter type could also be made general.