rendering for DiscreteExplicitPOMDP

[zsunberg]

It seems reasonable that someone might want to add visualization or something to their DiscreteExplicitPOMDP or MDP.

To do this, they might need to provide new methods for their specific problem. Perhaps we should provide a key to dispatch on.

struct DiscreteExplicitPOMDP{KEY, ...}
      ...
end

function DiscreteExplicitPOMDP(...; ..., typekey::Symbol=Symbol())
      ...
    return DiscreteExplicitPOMDP{typekey, ...}(...)
end

Then someone can write

render(m::DiscreteExplicitPOMDP{:mykey}, step)

[zsunberg]

Actually it might just be more straightforward to add a field called renderobject and do

render(m::DiscreteExplicitPOMDP, step) = render(m.renderobject, step)