Terrain resource

A terrain resource is a resource based on using the dominate, easily-accessible material from the surrounding landscape for building construction. There are three types of terrain resources:
 * 1) snow, in snow-covered tiles.
 * 2) sand, in desert tiles.
 * 3) sod in most other tiles.

Using a terrain resource represents using rammed earth construction techniques and digging down into the earth or snow to some degree. For this reason, terrain resources have some differences compared to other resource types:
 * It is only useful for constructing buildings, not equipment.  (It is not possible to build sand-pants.)
 * It cannot be be traded or transported to other tiles.
 * The supply is unlimited.
 * It requires no maintenance resource.
 * Construction time is generally longer, to reflect the added work of excavating the soil/sand/snow.

Some tiles like salt flats provide no terrain resources because they are not suitable for rammed-earth construction.