[WIP] Add small hills to mapgen

[arijust]

Summary

SUMMARY: Content "Add small hills to mapgen"

Purpose of change

Add some hills to wilderness, so map doesn't feel flat outside of cities.

Describe the solution

Add small hills with some vegetation.

Describe alternatives you've considered

Add one more Z-level to it.

Additional context

Please mention, if I am missing something like some parts for overmap settings.

[Amoebka]

The hill has perfectly square cross-sections on every z-level and no variants. It looks more like a ziggurat than a hill.
Hills should probably also be their own overmap terrain type on the same level as forests, plains and swamps, filled using a noise algorithm instead of randomly sprinkling them with no pattern.

[ghost]

Please don't try to turn Cata into Dwarf Fortress tier abomination of z-levels bonanza.

IMO the 'flat' terrain should be interpreted as ground level, no matter the elevation - which probably should be property for any such ground tier tile and any related mechanics like stamina/fuel drain or visibility modifies implemented from that, reserving up/down transitions to represent sheer drops, stairs etc.

[arijust]

Closing.
Other contributors can make these "in correct way" themselves.