Unit Map: added has_unit_at helper function

Just a small wrapper function to check whether a unit is present a certain location.

src/units/map.cpp

@@ -385,3 +385,8 @@ bool unit_map::has_unit(const unit * const u) const
 	}
 	return false;
 }
+
+bool unit_map::has_unit_at(const map_location& loc) const
+{
+	return find(loc) != end();
+}

src/units/map.hpp

@@ -394,6 +394,9 @@ public:
 	 */
 	bool has_unit(const unit * const u) const ;
 
+	/** Tests whether a unit exists at the given location. */
+	bool has_unit_at(const map_location& loc) const;
+
 private:
 	umap::iterator begin_core() const ;