Allow `Robodigger` to be placed on un-excavated `Tile`

[DanRStevens]

Allow Robodigger to be placed on un-excavated Tile which is adjacent to an excavated Tile.

In additional the the Tile of the mouse click, we can also place the Robodigger on said Tile, which is excavated upon initial placement, rather than on the already excavated Tile next to it. This would eliminate the need for the DiggerDirection window, and the Direction field of the Robodigger class. This both simplifies the user interface, and the programming interface.