Introduce "Unknown" state for Parity #19

The Parity objects require a third state (next to -1, 1). This third state, "Unknown" resembles that a particle will have a Parity but it is currently unknown if it is +1 or -1.
Next to those 3 intrinsic states of parities, some parities like G-parity or C-Parity are not always defined for particles. Hence these would be of type Optional[Parity]. The space inversion parity exists for all particles so this would be a non optional attribute.