feat: add is_carrying_minerals field for Unit class

Every unit has now a field is_carrying_minerals that will return True if the unit holds minerals, False otherwise.

docs/unit.rst

@@ -73,6 +73,9 @@ Unit
    .. autoattribute:: tile_position 
    .. autoattribute:: unit_type 
    .. autoattribute:: weapon_cooldown 
+   .. autoattribute:: is_carrying_minerals
+   
+      Returns if this unit is currently holding minerals
 
    .. attribute:: Unit.target