Allow for more complex, linear obstacles

[gerth2]

Imagine a field that looks like this:

We start at the O, with a goal at X

There's a solid chance we may have wall-like obstacles which act like this - we need new classes to describe these obstacles.

We may also need a "Lane" object that pushes the robot along a preferred linear path.

Set up a field that looks like this, and play around. Tweak obstacles till they work.