precalculate luminosities in `ImportedSourceLuminosityProbe` #150

Job	Run time
Build using Clang on macOS	1m 40s
Build using GCC on Ubuntu	2m 48s
Build using MinGW-64 on Windows	5m 22s
Build using Intel oneAPI on Ubuntu	3m 22s
Build using MSVC on Windows	3m 47s
	16m 59s

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