Added qrw::Coordinates::PtrCompLess for pointer comparision.

This struct can be used to order Coordinates* in a std::set.
Added Unit::canWalkSquare() method.
Added test for Coordinates::PtrCompLess to ensure std::set compatibility.
Made qrw::Coordinates immutable.
qrw::AStar now sorts it's temporary lists with qrw::Coordinates::PtrCompLess.
Brutally forced qrw::AStar privates to public in AStarTest.
Implemented qrw::AStar::clear() which cleans up working stuff.
Added test for standard copyctor for qrw::Coordinates.

This sounds stupid but ensures that an replacement of a copy constructor does not break compatibility with qrw::AStar which needs to copy coordinates.
Implemented qrw::Coordinates::operator==().
Added qrw::AStar dtor.
Implemented basics for AStar::getPath().

This includes precondition checks and return of an Path pointer on success. The acutal logic is still missing.