Add convex surface builder

[sethrj]

The "convex surface builder" is going to be the next building block above surfaces when constructing shapes, solids, etc. It's a single intersection operation on a list of convex surfaces, so it will be easy to reason about when formulating safety calculations, constructing building boxes, etc.

The expected use case will be to have a ConvexRegion class that can be reusable (for example doing rings and other partially negated regions). A Shape class, which "is-a" Object, i.e. something that can be turned into a Volume, will have a single convex region; a solid may have an inside and outside; etc.

// The building block of a shape or solid
class ConvexRegion
{
    virtual void build(ConvexSurfaceBuilder& add_surface) const = 0;
};

// A hemisphere as part of a shape or a solid
class HemisphereRegion : public ConvexRegion
{
    void build(ConvexSurfaceBuilder& add_surface) const final
    {
        add_surface(PlaneZ{0.0});
        add_surface(SphereCentered{radius_});
    }
};

// Primitive shape consisting of a single solid
class Shape : public Object
{
  public:
    virtual ConvexRegion const& interior() const;

    NodeId build(VolumeBuilder& vb) const final;
};

NodeId Shape::build(VolumeBuilder& vb) const final
{
    detail::ConvexSurfaceState css;
    css.transform = &sb.current_transform();
    css.object_name = this->label();
    ConvexSurfaceBuilder sb{vb.unit_builder(), &css};
    this->interior().build(sb);
    auto result = vb.add_csg_node(Join, std::move(css.nodes));
    // Save bounding zone and metadata for the shape
    return result;
}