Pyvalem

* Forbid whitespace within key-value pairs
* Allow whitespace, semicolon or commas to delimit states in StatefulSpecies

* Omit superscript "1" from singly-occupied AtomicOrbitals in AtomicConfiguration
* Omit semicolon delimiter in HTML output of StatefulSpecies states

- Allows `"e"` instead of `"e-"` as a valid electron formula string.

- Add GAMMA as a valid MolecularTermSymbol Lambda value
- import State ABC and all the states exceptions into the states namespace

* Some fraction of the codebase gets proper docstrings, documentation is changed so it allows for testing.
* All the states-related code gets refactored to a `pyvalem.states` subpackage. As a result, if the concrete state classes need to be imported on their own, this is done now by

from pyvalem.states import AtomicConfiguration
from pyvelem.states import MolecularTermSymbol

etc.

Relaxes the requirement to specify a single electron in an atomic orbital: "1s" and "1s1" are now both valid.