Set up environment

Set up environment to
- replace all rocks with supermines
- gathered mines are replaced randomly in next iteration (incl.
supermines)
- collision with mines collects mine and increments minesweepers
minesCollected
- collision with supermine removes minsweeper and supermine from
environment for that iteration
- added active bool to sweepers, mines and supermines
- added stat to record number of remaining sweepers at the end of an
iteration (incl. graph)