Add assets for forest and implemented age change in engine

src/main/java/engine/engineMain/AssetLoader.java

src/main/java/engine/engineMain/Assets.java

src/main/java/engine/engineMain/Handler.java

src/main/resources/objects/forest1.obj

src/main/resources/objects/forest10.obj

src/main/resources/objects/forest2.obj

src/main/resources/objects/forest3.obj

src/main/resources/objects/forest4.obj

src/main/resources/objects/forest5.obj

src/main/resources/objects/forest6.obj

src/main/resources/objects/forest7.obj

src/main/resources/objects/forest8.obj

src/main/resources/objects/forest9.obj

src/main/resources/textures/assets/forest.png

src/main/resources/textures/zones/grass.png