Renew the Terrain Modeling with ASCII files assignment

[wenzeslaus]

Update of content and perhaps used data is needed.

Code: https://github.com/ncsu-geoforall-lab/geospatial-modeling-course/blob/master/grass/terrain_modeling_ascii.html

Online: http://ncsu-geoforall-lab.github.io/geospatial-modeling-course/grass/terrain_modeling_ascii.html