[feat] Look into replacing the Object::Undefined() pattern with std::optional

[apaletta3]

Currently we use the ::Undefined() pattern to specify that an object is the None equivalent in python, but this is kind of cumbersome. You need to define an ::Undefined method for each object (half of our ostk-astro objects actually don't have it implemented) in order to get that behavior, and then when creating a composite object you have to always check memvar_->isDefined(); before you return it

It could be interesting to use std::optional and potentially even std::variant in a bunch of places to get more pythonic function signatures in cpp