Interactive mode

[joshgreaves]

We need to build in an 'interactive mode' for when we are developing in the unreal engine alongside python. The difference would be it doesn't run a build project in a different process, it just tries to connect to one running in the unreal editor, and you have control over what agent to expect etc.

Probably most easily implemented by either using the HolodeckEnvironment class and adding an 'interactive' flag, or creating a child class called HolodeckInteractiveEnvironment. The second would probably solve issues relating to passing in sensors, since in the other environment classes they are set after the init.