Mgv5/mgv7: Remove 'should make cave hole' feature
Remove ravine code

Remove large cave checks for air
Mgv5/mgv7:Add is_ground_content checks to 3d noise tunnels
More large caves
Shorten lines

Conf.example: Update mgv5 mapgen params
Mgv7: Lava caves only below -256

Add large pseudorandom caves below -256
De-link terrain level from water_level
Cavegen: Mgv5 large cave code

Add 'absolute value' option to noise map functions
Extend persistence modulation to 3D noise
Extend 'eased' option to noise2d_perlin* functions
Some noise.cpp formatting fixups

Add support for notify-on-decoration
Clean up mapgen constructors
Clean up mapgen.cpp code style somewhat
Remove trailing whitespace from some files

MapgenParams is no longer a polymorphic class, eliminating the need for messy and bug-prone reallocations.
Separation between the common and mapgen-specific parameters is now strongly defined.
Mapgen parameters objects are now properly encapsulated within the proper subsystems.

Cavegen: Prevent caves from occuring above ground level, and superfluous mixing of lava and water in caves