Set the amplitude of the compressed SLCs to be the mean amplitude of input stack

[scottstanie]

Here's what we're currently doing: https://github.com/isce-framework/dolphin/blob/d8fd3c8a04ae72ecdeacceee9a6298231515bba3/src/dolphin/phase_link/_compress.py#L33-L40

We're normalizing by the norm of the phase linking vectors in case we set the option to use the input SLC amplitudes. The current way leads to a compressed slc which is ...sort of similar amplitude to the input SLCs. but it doesn't have any real useful meaning now, so we might as well make it the SLC mean amplitude.