Revise objects to decrease their sizes

[375gnu]

If we reorder fields we can decrease memory usage because now a lot of memory is wasted by paddings.

[ajtribick]

I would also like to investigate whether or not it makes sense to split the DSO representation into the octree-relevant vs other details details, rather than having an inheritance hierarchy in DSO, along the lines of:

class DeepSkyObject
{
    Eigen::Vector3d position;
    float radius;
    float absMag;
    std::variant<std::unique_ptr<Galaxy>, std::unique_ptr<Globular>, ...> details;
}

(although we would probably want to implement the variant manually, since the std::variant will necessarily introduce padding between the discriminant and the pointer)

Would likely help with locality when walking the octree (could store the DeepSkyObject directly in the octree structure rather than adding indirection)