User subroutines for flocculation and hindering

Add user subroutines for user formulas for flocculation and hindering. Done during GAIA's hackathon 2023. @pablo.tassi @thomas.benson @nicolas.huybrechts

added module:gaia type:enhancement labels

closed with commit a682b31f

mentioned in commit a682b31f

mentioned in commit 5f9c1fe7

mentioned in commit 346018a4

mentioned in commit 96775cb3

mentioned in merge request !104