Less specific method signatures for default implementations

Sometimes very specific method signatures in POMDPs.jl make it more difficult to define methods for a problem definition, e.g. if isterminal(p::POMDP{S,A,O}, s::S) is defined in POMDPs.jl, then isterminal(p::MyPOMDP, s) will be ambiguous. This release removes overly specific definitions to make things easier.