Easier detector placement?

When the size of a detector changes then the position of that detector and all downstream detectors must be manually updated in each of the macro files so that the distances between detectors does not change. This can be quite tedious and time consuming. We have quite a few macros for various baseline FPF setups, I'm wondering if it would make sense if we instead specified the detector order and the separation between each detector in the geometry macro instead?

i.e. add commands like:
```
/det/detectorOrder FLArE FASERnu2 FASER FORMOSA
/det/detectorSeparation 4300 1200 1200 1200 mm
```

I'm not sure exactly how to implement this, but I feel like such a thing should be possible within GEANT4

审查现有的基线 FPF 几何宏文件，以了解当前如何指定探测器位置和下游间距。确定几何命令的入口点，然后确定如何表示探测器顺序和间隔，并在所有受影响的宏中验证结果；issue 未指定测试或具体文件。

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