Simplified and improved params loading. Commented new visionGraph code

- Simplified and improved params file loading. Used to require a number
of params to be present in advance, and a more complicated way of
specifying keys and values. Now it's straightforward. Still causes an
exception if no AI node picks up a param
- No longer load params file from disk repeatedly if it's already been
loaded
- Adapted ExperimentParams.properties and the exp-histparams.sh script
to the new params format
- Added comments to some of the new vision graph code (in Active and
Abstract AI nodes)