erltrek_galaxy: fix find_empty_sector/2 type bug

* Fix find_empty_sector/2 calling get_quad/2
  because find_empty_sector/2 called get_quad/2 with a wrong type
  (the first argument should have been #quadxy{})

* WIP: add typespecs
Rewrite erltrek_galaxy:find_empty_sector/1

* Use randomly chosen points in the galaxy for each object
* Set upper limit of 10000 attempts
  just in case when the random choice doesn't work
  (This is highly unlikely regarding the numbers of objects
   inside the galaxy)

* WIP: add typespec to the erltrek_galaxy module functions