Handling of labels in AtomicCell (#7)

* Defining is_representative in parser

* Improved descriptions and typing

* Defined AtomsState, Orbitals, CoreHole and HubbardInteractions

Deleted out Pseudopotentials

* Updated for Orbitals schema

* Improved schema, normalization, and descriptions of atoms_state.py

* Improved ModelSystem description

* Added condition for umn and orbitals_ref in HubbardInteractions

* Fixed OrbitalsState normalization

* Added feedback by @ndaelman-hu

* Added back n_ quantities for shapes