Lexicon update for the pybind module.

[NurAd-Din]

This is the first pull request I'm making for the new python bindings. This is also the first pull request I've ever made so please give some guidance on whether I wish add more content per pull request. This request includes some smaller classes relating to the major lexicon class exposure.

[JackTemaki]

This is also the first pull request I've ever made so please give some guidance

Sure, here some hints:

• change the title to reflect the actual changes / additions of your PR
• Change the description text and add a few more details what you are actually adding, although in this case there might be no useful information
• regarding the PR size: this is usually hard to determine. I would say here the addition is quite small, so if there is more useful things to add to that, I think you can make the PR a little larger. Just try to have this in blocks which cover related logic. As I am not familiar with RASR internals, it is hard for me to give guidance on what should be added at the same time.

[NurAd-Din]

I've changed the request to include all Lexicon class exposure for approval. I've also implemented some of the requested changes.

[hannah220]

Some class has a single default constructor .def(py::init<>()), some don't have a constructor at all.
If the C++ class has an implicit or explicitly defined default constructor (e.g., no arguments and does nothing special), and no other constructors, pybind11 will automatically expose it, and you don’t need to define .def(py::init<>()).
Can you make it consistent by only defining .def(py::init<>()) when it's necessary?

[NurAd-Din]

Previously requested changes implemented.