Add functions to p_tb_type for checking if it's root or primitive

[AmirhosseinPoolad]

This PR adds two new functions to the p_tb_type struct, is_root and is_primitive and changes the users to use these new functions instead of writing the logic every time. There's now less code duplication and the code a bit cleaner and more understandable.

I did have two questions about this part of code:

1. What is the meaning of a t_pb without a name? There's a couple of places that call is_primitive and see if the associated t_pb doesn't have a name which could be cleaned up further
2. Regarding the added TODO comment in cluster_legalizer.cpp:836 of this PR: Should it be an assert?

[AlexandreSinger]

LGTM, some minor comments on documentation.

Regarding t_pb's being unnamed, t_pb's are a heirarchical structure based on the pb_type. I am not exactly sure, but I think there may be pb_types in the heirarchy which are unnamed. You can try to investigate by seeing how t_pbs are constructed if you have the energy for it.