Unit test for `get_phase_lag` and `get_mean_phase_difference` functions

[swapsha96]

It requires creating two light curves (ref and CoI band) with the known phase lag. You can refer to Ingram 2016 paper and the tutorial in 'Phase Resolved Spectroscopy' folder in notebook repository which includes the equations.

[OussCHE]

hello @matteobachetti , I am willing to work on this issue. I am reading now the work of @swapsha96. If you have any advice, don't hesitate to tell me.