Hi guys!

I just want to let you know that we now have a native implementation of The Jacchia-Roberts 1971 atmospheric model in SatelliteToolbox.jl. You just need to use `master`

because I still need to do somethings before tagging a new version.

This model is supposed to be less accurate than Jacchia-Bowman 2008 and NRLMSISE-00. However, the computational burden is much lower (10x less than NRLMSISE-00 and something like 70x less than JB2008).

I managed to obtain some data from GMAT, but the tests are very limited. I will appreciate if anyone can help me to validate the implementation!

```
julia> jr1971(DatetoJD(2017,1,1,0,0,0), 45*pi/180, 0*pi/180, 300e3, 76.5, 72.5, 7)
JR1971_Output{Float64}
nN2: Float64 4.0269432130021164e13
nO2: Float64 1.8040704522884658e12
nO: Float64 4.976154354226309e14
nAr: Float64 4.914191344437922e9
nHe: Float64 1.0731503643725797e13
nH: Float64 0.0
rho: Float64 1.526144428631946e-11
T_exo: Float64 871.4209785497403
Tz: Float64 858.5927825073542
```

The documentation is:

```
function jr1971(JD::Number, glat::Number, glon::Number, h::Number, F10::Number,
Compute the atmospheric density using the Jacchia-Roberts 1971 model.
Args
≡≡≡≡≡≡
• JD: Julian day.
• glat: Geodetic latitude [rad].
• glon: Geodetic longitude [rad].
• h: Altitude [m].
• F10: 10.7-cm solar flux [10⁻²² W/(M² Hz)].
• F10ₐ: 10.7-cm averaged solar flux, 81-day centered on input time.
• Kp: Kp geomagnetic index (with a delay of 3 hours).
Returns
≡≡≡≡≡≡≡≡≡
An instance of the structure JR1971_Output with the computed values.
```