improve doc strings

[Daafip]

should resemble: https://ewatercycle.readthedocs.io/en/latest/autoapi/ewatercycle/base/forcing/index.html with examples etc in code

[Daafip]

• [ ] check code for
• [ ] add example of use per class
• [ ] explain thought process of implementation
• [ ] improve formating

[Daafip]

Better description of model origins and that its most like HBV-3 from https://urn.kb.se/resolve?urn=urn:nbn:se:smhi:diva-5738, with HBV-96 being a more modern version: https://doi.org/10.1016/S0022-1694(97)00041-3 but containing 12 parameters insetead of the 9 used here