Magnetic variation is incorrect, depending on the date

[xchrishawk]

The displayed magnetic headings for runways can be incorrect. The underlying issue is that the simulator changes the magnetic variation depending on the currently selected "simulated" date.

The proposed solution is:

• Make all headings in the data index true headings
• Add magnetic variation databases for the relevant eras supported by DCS
• Add an option in the application settings to select the simulation era for magnetic variation calculation purposes
• Calculate the magnetic heading based on the magnetic variation, rather than just storing it statically
• Also add an option to support displaying true headings