ExecutableNode improvements

This is a bit of a tidy up of the Executable code so it's in slightly better shape for someone to take on #109. The meat of it is backing out of an awkward multiple inheritance setup where any Node subclass could inherit an additional Executable interface class to become Executable - I forget why I pushed for it at the time but it's since become clear that it wasn't the right way to go, hence I'm tidying up my mess. There is now a single ExecutableNode base class which sits nicely with the DependencyNode and ComputeNode classes, and things are simpler all round.

My final commit outlines some work that I believe is pretty critical in the ExecutableNode::Task class - Lucio and I talked about it briefly via mail but rather than implement it I've left it as a todo item for now. We should discuss further and implement early on when doing #109.