footprint calculation Kljun et al. (2015)

[holukas]

code: https://footprint.kljun.net/index.php

[inkenbrandt]

Also see examples in:

• https://github.com/mkorkiak/Footprints_Kljun15_KM2001
• https://github.com/holukas/diive/issues/new
• https://github.com/OzFlux/OzFlux-footprint