Better continuum pre-processing and/or enable blasé emulator continuum warping by default

[gully]

Currently our default is to turn off continuum warping. We should consider turning this back-on, probably by following the example in InstrumentModel. In the long term, we may consider moving to GPs, but for now Polynomials are good enough.

Related issue-- we should do better pre-processing of the input precomputed synthetic model spectra, maybe demand that no peak is greater than say 0.1% above 1.