rendering for DiscreteExplicitPOMDP

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)
```

Locate the definition and constructor for DiscreteExplicitPOMDP, then review how model-specific methods are currently dispatched. Determine whether a type key is the intended extension point for rendering or other custom behavior. Done means the API design is agreed and the relevant dispatch behavior is implemented with coverage for the proposed use case.

Written by the indexing model from the issue text.