Observable control plots

[SBouchard01]

1. Add option to propagate a factor on the data error in plot_emulator_residuals

This would allow to correct the mock error by a factor to get an idea of how the emulator error compares to the the true data error

2. Add a plot to get the emulator Z-scores distribution histogram (see fig.5 of https://arxiv.org/pdf/2309.16539)

@epaillas is this relevant ? 🤔