Renaming psd-related variables to avoid confusion

[tsunhopang]

The load_psd function is modified such that

1. If no PSD file is provided, it automatically fetches the preset ASD files and squares them to get the PSD
2. If a PSD file is provided, treat the values in the file as PSD without squaring

This is to avoid confusion with the current implementation of the load_psd function, as it treats the inputs as ASD files.

[tsunhopang]

I also suggest to move the load_psd away from inject_signal and have it run during the initialization.

[kazewong]

Regarding moving the load_psd, I think it is more general to keep a separate method to load the actual data as it is now, since in the case of fetching real data, the PSD/ASD file is never fetched in the way load_psd is called.

The rest of the PR looks good to me, merging to main.

[tsunhopang]

In production, we usually use BayesLine PSD as input rather than re-estimating the PSD using the Welch method. So the load_psd is actually more realistic for production run.