make single public function for havengetallen

[veenstrajelmer]

• [x] single public function
• [x] use this also in gemgetij, so there is no dependency on files
• [x] properly format docstring of kenmerkendewaarden.gemiddeldgetij.gemiddeld_getij_av_sp_np, now it looks messy in sphinx docs
• [x] fix havengetallen so we can use old results/figures as validation again >> not possible, also oldest functions in hatyan and old datasets give the same results, something probably has changed a long time ago.