Discrete movement can't trigger Reward/Quit for touching block type by walking up to the block

[DaveyBiggers]

Interesting philosophical issue...
Discrete movement won't allow an agent to touch the side of a block, since the agent can't co-exist in the same grid square as the wall. This is correct.

QuitFromTouchingBlockType / RewardForTouchingBlockType won't fire if the agent is merely adjacent to the block, since it's not actually touching the block. This is also correct.

But this means a discrete agent has no way to trigger the reward/quit - which is not an uncommon use case.

(This was raised previously in #103, where it was specifically a problem with the MazeDecorator - we worked around it in that case, but I think the general case is still worth some thought.)