Support Mack 1993 correlation tests across valuation years

[genedan]

Since this is used as a test for the reasonableness of the development technique, it should be accessible to the user shortly after triangle importation.

raa = cl.load_sample("raa")
print(
    "Correlation across valuation years? ",
    raa.valuation_correlation(p_critical=0.1, total=True).z_critical.values,
)

https://chainladder-python.readthedocs.io/en/latest/tutorials/development-tutorial.html#testing-for-violation-of-chain-ladder-s-assumptions

[genedan]

Nothing fancy, but I added a "Diagnostics" option for the value dropdown which displays a separate widget containing the tests:

Since this issue didn't say anything about being able to alter the parameters, I'm creating a separate issue for that.