Add V7 caves to Valleys mapgen.

[duane-r]

This adds a template for the CaveV7 class, so that Valleys mapgen can
use it without duplicating code.

[paramat]

👎 An option to use mgv7 type large pseudorandom caves and 3D noise tunnels is okay by me, but instead use CaveV5 in cavegen.cpp for the large caves, it is identical to CaveV7 but it removes the unnecessary ridge heightmap, and CaveV5 has already been changed to be usable with any mapgen, see how mgflat and mgfractal use it.

You can still refer to the option as 'mgv7 caves' to avoid confusion, and because the 3D noise tunnels are still from mgv7, just at the end of 'generateV7Caves()' call CaveV5 instead.

This way you won't need to change CaveV7 in cavegen.cpp at all.

[duane-r]

instead use CaveV5 in cavegen.cpp

Cool. I will rework that. I think I'll change the flags to V5 though, just to avoid confusing me.