Projectors (FOR REVIEW)

please review the projectors and you also may merge it as soon as we agreed on everything.

TODO: Autogenerate ProjectionMethods. Any suggestions which methods we should expose to the user?

* FirstOrderStatistics (Mean,Min,Max etc).

what else?

@dietzc - after discussion in person, we realized that the two stages here are the exact same pattern as the `Threshold`. So you should be able to split the API similarly and auto-generate `Project` methods for appropriate ops, which will then simplify their use.