Merge and split fleets

Requirements for merging fleet 1 and fleet 2:

• Fleet 1 and fleet 2 are at the same position.
• Fleet 1 and fleet 2 are both stoped, ie are not performing any command.

If you merge a space fleet and a ground fleet the resulting fleet will have ground status.

Requirements for splitting a fleet:

• The fleet is stopped.
• The fleet consists of more than one unit.
• If the fleet contains planetary units and you want to split them off, the fleet has to have ground status.