added alternative way to do makePOMDPGridworld
added alternative way to do makePOMDPGridworld

jsalvatier/webppl-agents,jsalvatier/webppl-agents,agentmodels/webppl-gridworld,agentmodels/webppl-agents,jsalvatier/webppl-agents,agentmodels/webppl-gridworld,agentmodels/webppl-gridworld