Feature - linear momentum integrals in LAO basis

**Short description** Adding python bindings for the linear momentum and current density integrals in a London Atomic Orbital (LAO) basis. **Related issues** - #1076 **To do** - [x] Implement linear momentum operator bindings for London basis. - [ ] ~Implement current density operator bindings for London basis~ - [x] Add examples to `London-integrals.ipynb`