Support multiple distance units

[dkastl]

To build a software, that works also for people with non-metric distances, we should provide some configuration.

Eventually this could be best configured in global GTT settings.

Possible units:

• 'meters' or 'm'
• 'feet' or 'ft'
• 'kilometers' or 'km'
• 'nauticalmiles' or 'nm'
• 'miles', 'statutemiles', 'mi', or 'sm'
• 'degrees' or 'deg'
• 'radians' or 'rad'

We need to see, which make sense and can be supported with small effort.

Users should be able to see, which unit the numbers should have.