Add a distribution plot for total chi²

[Zaharid]

We would like to have plots similar to

https://vp.nnpdf.science/hB56KrDqTmW_zmchLLlTOg==/figures/experiments0_experiment0_NNPDF31_nlo_as_0118_plot_chi2dist.pdf

that work not only for a single dataset, but for arbitrary aggregates of datasets, including in particular the total chi².

Bonus points if we can have kde plots as well (

nnpdf/validphys2/src/validphys/plotutils.py

Line 392 in d5f6751

def kde_plot(a, height=0.05, ax=None, label=None, color=None, max_marks=100000):

).

Note that this will need to be modified to work for #651 .

[siranipour]

Hmmm I wonder why you want these plots :P no worries boss, will get to work on this.