Add kurtosis and other metrics to evaluate quality of error standardization

[rhugonnet]

See #442, closed for now.

Would need to come with a documentation page specific to it.

Here's a bit of code to do the kurtosis calculation:

from scipy.stats import kurtosis
kbef = kurtosis(dh_arr, nan_policy="omit")
kaft = kurtosis(z_dh.flatten(), nan_policy="omit")
print("Excess kurtosis before standardization: {}".format(kbef))
print("Excess kurtosis after standardization: {}".format(kaft))