Add a LoadPlanet method to FGFDMExec

[bcoconni]

Following an idea from @seanmcleod (see #908 (comment)), this PR introduces a new method FGFDMExec::LoadPlanet() which can be used to load a planet definition such as tests/moon.xml:

jsbsim/tests/moon.xml

Lines 1 to 7 in 8c13fd6

	<planet name="Moon">
	<equatorial_radius unit="KM">1738.1</equatorial_radius>
	<polar_radius unit="KM">1736.0</polar_radius>
	<rotation_rate unit="RAD/SEC">2.66167E-6</rotation_rate>
	<GM unit="M3/SEC2">4.9E12</GM>
	<J2>2.027E-4</J2>
	</planet>

As Sean explained, this feature allows to manage independently the aircraft model and the planet definition file on which the simulation should take place.

The programs JSBSim.exe and JSBSim.py have been updated accordingly. A script such as ball_chute.xml can therefore be run on the Moon using the following command line:

> JSBSim --script=scripts/ball_chute.xml --planet=tests/moon.xml

The ball needs 107.8 sec to fall from an height of 10000 ft on Earth and 329.6 sec to fall from the same height on the Moon. One might notice that the ratio of these 2 duration is not equal to $\sqrt{6}$ but the script deploys a parachute at an height of 5000 ft and we currently cannot force the Moon atmosphere to be the vacuum 😉

[codecov]

Codecov Report

Merging #913 (b91e792) into master (8c13fd6) will decrease coverage by 0.04%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #913      +/-   ##
==========================================
- Coverage   23.14%   23.11%   -0.04%     
==========================================
  Files         167      167              
  Lines       19603    19631      +28     
==========================================
  Hits         4538     4538              
- Misses      15065    15093      +28     

Impacted Files	Coverage Δ
src/FGFDMExec.cpp	39.78% <0.00%> (-1.54%)	⬇️
src/FGFDMExec.h	17.14% <ø> (ø)

[seanmcleod]

Looks good.

[jonsberndt]

Awesome addition! Jon