Lua functions for spawning landed ships and querying ground positions

As requested in issue #1378, this adds a Lua function which spawns a ship landed on a planet:

```
/*
 * Function: SpawnShipLanded
 *
 * Create a ship and place it landed on the given <Body>.
 *
 * > ship = Space.SpawnShip(type, body, lat, long)
 *
 * Parameters:
 *
 *   type - the name of the ship
 *
 *   body - the <Body> near which the ship should be spawned
 *
 *   lat - latitude in radians (like in custom body defintions)
 *
 *   long - longitude in radians (like in custom body definitions)
 *
 * Return:
 *
 *   ship - a <Ship> object for the new ship
 *
 * Example:
 *
 * > -- spawn 16km from L.A. when in Sol system
 * > local earth = Space.GetBody(3)
 * > local ship = Space.SpawnShipLanded("viper_police", earth, math.deg2rad(34.06473923), math.deg2rad(-118.1591568))
 *
 * Availability:
 *
 *   TBD
 *
 * Status:
 *
 *   experimental
 */
```