use posterior::gpdfit and posterior::qgeneralized_pareto()

[avehtari]

Since we do use posterior package elsewhere, replace gpdfit() and qgpd() with posterior::gpdfit and posterior::qgeneralized_pareto()

This will make maintenance easier. For example, there is a PR for posterior::gpdfit() to make it more robust and posterior::qgeneralized_pareto() already is more robust than qgpd()

This is related to #249, but timing is due to the fix PR for posterior::gpdfit()

[avehtari]

If it has been exported, then someone might be using it? Should I put it back and define it with posterior::qgeneralized_pareto()?

[avehtari]

At least in github only I and Juho Timonen have used loo::gpdfit, so I guess it can be removed

[jgabry]

Good point. Yeah you can just replace the bodies of the functions with calls to the posterior versions.

[jgabry]

At least in github only I and Juho Timonen have used loo::gpdfit, so I guess it can be removed

OK in that case, we can try removing it and we will find out in the reverse dependency checks whether any packages are using it

[jgabry]

The error now is because apparently these functions aren't exported from posterior, they're just internal functions. That would be easy to change but it would require doing a CRAN release of posterior.

[avehtari]

my bad, I guess this needs to wait for the next posterior release

[jgabry]

There is also #290 from @VisruthSK that is waiting to be merged until there is a new release of posterior