Calculate Kernel Density Estimation (KDE) for spatial data.
The algorithm is inspired by the tool 'Heatmap' from 'QGIS'. The method is described by:
Hart, T., Zandbergen, P. (2014) ,
Nelson, T. A., Boots, B. (2008) ,
Chainey, S., Tompson, L., Uhlig, S.(2008) .