Add (estimated) variable staff costs to aircraft profitability

[JumboShrimp]

Right now, profitability of aircraft excludes all of the fixed costs.

Flight crews are more in the variable category.  It would be helpful for these costs to be added to aircraft profitability.

As of now, players are thinking their planes are making money, and are wondering why the company is losing money.  This feature will give a partial answer.

Possible future extension of this (in the future): allocate these variable staffing costs to flights.

[TheGrew]

I think this would be helpful. I made a fleet transition purely based on reducing my crewing requirements (which is very high based on my location).