rewrite expectedvalue. still need to test a bit more
rewrite expectedvalue. still need to test a bit more

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