SP Terrain challenge

[Narwan]

The following are from my observations in the game. I do not know how these are all coded with regards to cover. But here are my suggested changes:

* Streams give worse cover than clear terrain.
* Shellholes should be just one category below trenches/foxholes.
* the combination of rough and wooded gives the best cover from harm of all terrain types (possibly matched by stone building rubble which I place as the second best terrain for cover)
* Beach sand has worse cover than clear terrain


Narwan