Repetitive SeismicObservable Fields

[bvanderbeek]

All the SeismicObservable types share the same six fields, (1) Phase, (2) Forward, (3) source_id, (4) receiver_id, (5) observation, and (6) error. The following would be cleaner,

struct SeismicObservableAttributes{P, F, S, R, T}
    Phase::P
    Forward::F
    source_id::S
    receiver_id::R
    observation::T
    error::T
end

and then a new SeismicObservable is simply

struct NewSeismicObservable <: SeismicObservable
    attributes::SeismicObservableAttributes
end