Plot function: no auto *10^i offset to multiple curves and optional R*Q^4 format

[paracini]

https://github.com/EasyScience/EasyReflectometryLib/blob/master/src/easyreflectometry/plot.py

Currently the plot function automatically scales the intensity of multiple reflectivity curves by a power of 10 to make the different curves more visible.

This should be off by default. Instead the function should take an optional arguments that specifies whether or not to offset the curves.
Following the same approach, a different optional argument should enable plotting the reflectivity curves as a function of R*Q^4