Move seat variables into better location in hierarchy

Desired capability or behavior.

Currently these variables:
Aircraft.CrewPayload.Design.NUM_SEATS_ABREAST_BUSINESS
Aircraft.CrewPayload.Design.NUM_SEATS_ABREAST_ECONOMY
Aircraft.CrewPayload.Design.NUM_SEATS_ABREAST_FIRST
Aircraft.CrewPayload.Design.SEAT_PITCH_BUSINESS
Aircraft.CrewPayload.Design.SEAT_PITCH_ECONOMY
Aircraft.CrewPayload.Design.SEAT_PITCH_FIRST

Are in a strange location in the hierarcy as they describe the aircraft configuration, not it's loading.

Suggest we move them to
Aircraft.Fuselage.NUM_SEATS_ABREAST_BUSINESS
Aircraft.Fuselage.NUM_SEATS_ABREAST_ECONOMY
Aircraft.Fuselage.NUM_SEATS_ABREAST_FIRST
Aircraft.Fuselage.SEAT_PITCH_BUSINESS
Aircraft.Fuselage.SEAT_PITCH_ECONOMY
Aircraft.Fuselage.SEAT_PITCH_FIRST

So that they sit alongside the SEAT_WIDTH variables.

This issue should include a discussions on the following:

• Should we change Aircraft.CrewPayload.Design.NUM_BUSINESS_CLASS etc to be totals of the number of seats rather than the number of passengers on the 'design' mission. Maybe these should also be moved to Aircraft.Fuselage.* if we choose to do this.
• Logic in preprocessors might need adjusting to check loading against the number of seats in the aircraft, not the number of passengers flown on the 'design' mission.

Suggested implementation.

Move them in variables.py and variable_mata_data.py then update everywhere they are referenced in the code and docs.

Start in variables.py and variable_mata_data.py to understand the current definitions, then search the code and documentation for all listed variable references. Resolve the requested hierarchy move and the open questions about seat totals and preprocessor loading checks; done means the definitions, references, preprocessors, and docs consistently use the chosen locations and semantics.