Add ability to rover to mine out an adjacent grid point

[hoylemd]

it should lock the rover in a position and facing, and for an appropriate time, add a mining effect. Grid points underground should maintain an "amount mined" variable, representing how much it has been mined out. This variable should decrease until the end of the timer, when it should be zero. Resources from mining should also increase and decrease in time with mining. Once mining is complete, the meshes for that grid point and affected adjacent ones should be updated to reflect the modified cave layout.