SymbolicDiscreteCallback and SymbolicContiniuousCallback have different affect field names

[TorkelE]

Not a bug really, but an internal oddity that is probably a mistake.

The affects field is called affect for ``. Since it can hold several affects, affects should probably be used.

struct SymbolicDiscreteCallback
    # condition can be one of:
    #   Δt::Real - Periodic with period Δt
    #   Δts::Vector{Real} - events trigger in this times (Preset)
    #   condition::Vector{Equation} - event triggered when condition is true
    # TODO: Iterative
    condition::Any
    affects::Any

struct SymbolicContinuousCallback
    eqs::Vector{Equation}
    affect::Union{Vector{Equation}, FunctionalAffect}

Might be intentional, and have no impact on stuff. However, since I noticed I figured I should point it out. Feel free to close this issue whenever you want.