🔧 Change SGP4 structs to use Parameters.jl

This toolbox will be very handy when implementing the SDP4 bits
into the propagator, since the structures will have a lot of
parameters.

It was also verified a little performance improvement after
switching from the old approach to Parameters.jl.

src/orbit/propagators/sgp4.jl

@@ -48,11 +48,6 @@ export sgp4_init, sgp4!
 #                                  Overloads
 ################################################################################
 
-# Serialization of the arguments in SGP4_GravCte.
-function getindex(sgp4_gc::SGP4_GravCte, ::Colon)
-    sgp4_gc.R0, sgp4_gc.XKE, sgp4_gc.J2, sgp4_gc.J3, sgp4_gc.J4
-end
-
 # Copy for SGP4_Structure.
 function Base.copy(m::SGP4_Structure)
     SGP4_Structure([ getfield(m, k) for k = 1:length(fieldnames(m)) ]...)
@@ -63,19 +58,6 @@ function Base.deepcopy(m::SGP4_Structure)
     SGP4_Structure([ deepcopy(getfield(m, k)) for k = 1:length(fieldnames(m)) ]...)
 end
 
-# Serialization of the arguments in SGP4_Structure.
-function getindex(sgp4d::SGP4_Structure, ::Colon)
-
-    sgp4d.t_0, sgp4d.n_0, sgp4d.e_0, sgp4d.i_0, sgp4d.Ω_0, sgp4d.ω_0, sgp4d.M_0,
-    sgp4d.bstar,sgp4d.a_k, sgp4d.e_k, sgp4d.i_k, sgp4d.Ω_k, sgp4d.ω_k,
-    sgp4d.M_k, sgp4d.n_k, sgp4d.all_0, sgp4d.nll_0, sgp4d.AE, sgp4d.QOMS2T,
-    sgp4d.β_0, sgp4d.ξ, sgp4d.η, sgp4d.sin_i_0, sgp4d.θ, sgp4d.θ2, sgp4d.θ3,
-    sgp4d.θ4, sgp4d.A_30, sgp4d.k_2, sgp4d.k_4, sgp4d.C1, sgp4d.C3, sgp4d.C4,
-    sgp4d.C5, sgp4d.D2, sgp4d.D3, sgp4d.D4, sgp4d.dotM, sgp4d.dotω, sgp4d.dotΩ1,
-    sgp4d.dotΩ, sgp4d.isimp, sgp4d.sgp4_gc
-
-end
-
 ################################################################################
 #                                  Constants
 ################################################################################
@@ -135,7 +117,7 @@ function sgp4_init(sgp4_gc::SGP4_GravCte{T},
                    bstar::Number) where T
 
     # Unpack the gravitational constants to improve code readability.
-    R0, XKE, J2, J3, J4 = sgp4_gc[:]
+    @unpack_SGP4_GravCte sgp4_gc
 
     # Constants
     # =========
@@ -308,12 +290,8 @@ values in `sgp4d` will be modified.
 """
 function sgp4!(sgp4d::SGP4_Structure{T}, t::Number) where T
     # Unpack variables.
-    t_0, n_0, e_0, i_0, Ω_0, ω_0, M_0, bstar, a_k, e_k, i_k, Ω_k, ω_k, M_k, n_k,
-    all_0, nll_0, AE, QOMS2T, β_0, ξ, η, sin_i_0, θ, θ2, θ3, θ4, A_30, k_2, k_4,
-    C1, C3, C4, C5, D2, D3, D4, dotM, dotω, dotΩ1, dotΩ, isimp, sgp4_gc =
-    sgp4d[:]
-
-    R0, XKE, J2, J3, J4 = sgp4_gc[:]
+    @unpack_SGP4_Structure sgp4d
+    @unpack_SGP4_GravCte   sgp4_gc
 
     # Time elapsed since epoch.
     Δt = t-t_0

src/types.jl

@@ -333,7 +333,7 @@ Gravitational constants for SGP4.
 * `J4`: The fourth gravitational zonal harmonic of the Earth.
 
 """
-struct SGP4_GravCte{T<:Real}
+@with_kw struct SGP4_GravCte{T<:Real}
     R0::T
     XKE::T
     J2::T
@@ -345,7 +345,7 @@ end
 Low level SGP4 structure.
 
 """
-mutable struct SGP4_Structure{T<:Real}
+@with_kw mutable struct SGP4_Structure{T<:Real}
     # TLE parameters.
     t_0::T
     n_0::T