add gg,ghg,glg from PASTAS module

[TobiasMulder]

Hi,

it would be great if the statistics section is expanded with the ghg, gg and glg.
This can easily be incorporated with pastas:
https://pastas.readthedocs.io/en/latest/api/generated/generated/pastas.stats.dutch.html
This gives a bit more insight than just the mean, quantiles etc.