Support different reco toolkits for near & far in extrapolation

[chenel]

Currently the extrapolation machinery assumes the same reco toolkit (dlp, pandora, etc.) will be used by Near and Far. This is unlikely ever to be true, and anyways we will want to study what difference it makes.

In practice, extending this to be more general will require several steps:

• updating the various Extrap classes (most notably NumuExtrap, but while we're at it also TrivialExtrap for good measure) to support separate RecoTypes for ND and FD, and to pass them along to the relevant Loaders instance in their constructor (e.g., here)
• overhauling the IPredictionGenerator::Generate() interface to accept different RecoTypes for FD and ND, then propagating that to its derived classes (PredictionNoExtrap and PredictionGenerator3Flavor, at least).
• Passing the RecoType onto the extrap instance in the PredictionGenerator's constructor (e.g., here)