Implement df::coord iterator and cuboid class

[Bumber64]

Implement an iterator that operates on all df::coords within a cuboid box. Iterates in the direction of x/y/z provided (rather than always min to max.) By default iterates per z-level, but has an option to iterate in the z direction first.
Closes#4596

Cuboid class for defining a cuboid of tiles. Can test if a coord is inside itself. Can be expanded to contain a given coord. Has a method to call the above iterator using the defined cuboid.
Closes#4595

Also update plugins to use plant_type enum attrs, since we're removing the cuboid struct from plugins/plant.cpp in this PR anyway.

[Bumber64]

Need to apply this to existing functions (e.g., Units::isUnitInBox and Units::getUnitsInBox, as well as plugins that iterate map tiles).

[Bumber64]

Should a class with only public members more properly be a struct, or is it weirder to have a struct with member functions?

[myk002]

Should a class with only public members more properly be a struct, or is it weirder to have a struct with member functions?

I don't have an opinion one way or the other here. I'd say do whatever feels natural.