change default behaviour of randomized agent positions

[ufuk-cakir]

The default behaviour in the Baseline Julia POMDP package is that the agent position is not randomized and always starts at the same position

https://github.com/JuliaPOMDP/RockSample.jl/blob/4b6f9113e2dfbecb959d372e2913cefe2a348e3c/src/RockSample.jl#L42

It would be good to also have this here.