AP_Mission: Command and waypoint handling library.

The library:

- Reads and writes MAVLINK commands to and from EEPROM.
- Provides the next nav and associated non_nav command(s) on request.
- Performs error checking on waypoint values.
- Keeps track of the index for the current nav command and non_nav_commands.
- Contains state variables to help control the mission and progression through
commands.