#408 smoothing routine for low pass filtering

[Karl Ratzsch]

It´d be great to have a smoothing routine that calculates a running average with a given bandwidth, possibly with different kernels (boxcar, gaussian, ..)?

A gaussian filter is already possible using "kdensity" with suitable weighting (ordinate value/sampling frequency), but i´m not sure how the weighting should be towards the ends of the abscissa.