Rewrote the map parser

[ChristopherCanfield]

• The map parser can now use either the initial tileset, with craters and rubble classified as terrain, or the final tileset, with craters and rubble classified as stationary elements.
• The order that the tilesets are added to a map no longer matters, because the first gid for each tileset is now accounted for.
• Updated json-simple to 3.1.1
• Bubolo now uses Java 15.