Add UML PDF and adjust World display

Add uml_diagram.pdf and a .gitignore entry to ignore the TeX source.
Remove unused <atomic> include, shorten the console separator, and add
a comment describing default player position values in World::display.

.gitignore

@@ -0,0 +1 @@
+uml_diagram.tex

include/World.h

@@ -14,7 +14,7 @@ class World {
     int getSurfaceLevel ( int x, int y ) const;
     int mine ( int x, int y );
 
-    void display ( int p1x = -1, int p1y = -1, int p2x = -1, int p2y = -1 ) const;
+    void display ( int p1x = -1, int p1y = -1, int p2x = -1, int p2y = -1 ) const; // player positions initialized to -1 -> outside of surface grid
 
     int getSizeX () const { return _sizeX; }
     int getSizeY () const { return _sizeY; }

src/World.cpp

@@ -4,7 +4,6 @@
 #include <random>
 
 #include <algorithm>
-#include <atomic>
 
 World::World ( int x, int y, int z )
     : _sizeX ( x ), _sizeY ( y ), _sizeZ ( z ),
@@ -94,7 +93,7 @@ void World::display ( int p1x, int p1y, int p2x, int p2y ) const {
                 std::cout << std::setw ( 4 ) << val;
         }
         std::cout << "\n";
-        std::cout << "=================================\n";
+        std::cout << "=========================\n";
     }
 }