Currently, some are 500x500

In board.gd, initialize the 3D array as an array of empty block.gd's. When an area is covered by platform, set the variable platform to be true

Also add some polish to the visuals

1. Change the current layer for every single node in platform path to be "not available". Solves #11
2. Make every node in platform path on next layer to be "available". Need to make sure it defaults to unavailable
3. Make sure we check for availability when laying
4. When laying next block, go up to the first available layer and place

If don't choose anything, on end turn, add the money (remove this from player script and move to game_controller.gd)

Related to #3