Optimize calls to `SGPropertyNode::getNode` in `FGAtmosphere::Calcula… #1663

Job	Run time
MSIS code validation (macos-latest)	14s
XML validation	19s
MSIS code validation (ubuntu-latest)	8s
C/C++ build (MSVC) (ON)	6m 41s
C/C++ build (MacOSX)	6m 13s
C/C++ build (MinGW) (ON)	3m 53s
C/C++ build (MinGW) (OFF)	6m 0s
C/C++ build (Linux) (ubuntu-24.04, OFF, ON, OFF, ON)	4m 26s
C/C++ build (Linux) (ubuntu-24.04, ON, OFF, OFF, ON)	4m 22s
C/C++ build (Linux) (ubuntu-24.04, OFF, OFF, OFF, ON)	5m 4s
C/C++ build (MSVC) (OFF)	5m 2s
C/C++ build (Linux) (ubuntu-24.04, ON, ON, OFF, ON)	4m 17s
C/C++ build (Linux) (ubuntu-22.04, OFF, OFF, ON, OFF)	6m 13s
Build Python Module from Source (macos-latest)	3m 43s
Build Python Module from Source (windows-latest)	4m 17s
Build Python Module from Source (ubuntu-latest)	3m 9s
Matlab S-Function (windows-latest)	7m 24s
Matlab S-Function (ubuntu-22.04)	3m 17s
Matlab S-Function (macos-14)	3m 35s
Build Python wheels	0s
Build Windows installer	39s
Deploy Rolling Release	13s
Deploy Documentation	0s
Deploy Stable Release	0s
	1h 19m 9s

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