Use of accessor methods

[cgarling]

I think we should try to use accessor methods wave and flux everywhere possible. One reason for this is represented in the SpectrumResampler implementation: it encapsulates a Spectrum and has valid flux and wave methods, but the current version of redden! will not work because it relies on the existence of a spec.flux field rather than the flux(spec) method.

[icweaver]

Yes to all of this. Addressing in #24

[icweaver]

Note to future readers: Switched from @. obj.field to foo = field(obj); @. foo to avoid broadcasting shenanigans. Could also escape with @. $field(obj), but why be cryptic when we don't need to be.

Example: c9b31f7