Include individual action costs in solution

[sea-bass]

When you solve a problem using PDDLStream, you get access to all the individual actions and the total cost of the plan as the first and second elements of the solution tuple.

However, it would be really convenient to also get the cost of each action individually as part of this solution.

The easiest thing to do would be for the Action namedtuple to include name, args, and an additional cost field.

Right now there is the workaround of re-evaluating each cost function for each action in the plan, but it would be nice to avoid having to do this.