1. Replace lpsolve APIs with the scipy milp APIs. The new implementation potentially have a slightly better performance (due to vectorization, less data transfer, comparable solver performance, etc.) and improved readability.
2. Decouple the MILP solving step (as _solve_clar_milp ) from the MILP formulation step. The motivation is to avoid unnecessary computation. The original approach includes molecule analysis (specifically `get_aromatic_rings`) into the recursive calls. However, this is not necessary, as molecules are just copied and not modified at all. Therefore analyzing once is enough.

Co-authored-by: Hao-Wei Pang <45482070+hwpang@users.noreply.github.com>