move_forward has confusing behavior

[danielgordon10]

https://github.com/facebookresearch/House3D/blob/master/House3D/core.py#L196

Because the agent's position and the camera location and orientation are coupled, move_forward has unexpected behavior. If the camera's pitch is non-zero, then the agent will move less than the expected amount in the "forward" direction. It will instead move the amount projected onto the [x, 0, z] vector, which is not unit length if the y is nonzero.

To fix this, there are two options: One, you zero out the y-value and renormalize the vectors, giving the expected magnitude of motion. I have a version that does this, but I haven't set up a clean commit for a pull request. I can do this later if you would like.

Two: you put a comment in the code explaining this behavior as I would argue it is not expected. I would also argue it is not desired, and this solution, while easier, is less useful.