Add-API for UserDemand tables

[SouthEndMusic]

It has turned out on several occasions that the UserDemand tables (specifically the UserDemand \ time table), are hard to understand in terms of what they allow the modeller to do. In essence this table define mappings of the form (node_id, priority, time) -> demand, but it is not a priori clear which column is an input of this mapping an which is an output. Hence the erroneous conclusion that priority can be time dependent (for instance here).

These confusions arise in part because the tables are in long format, but we probably are not going to deviate from that. To make defining input more intuitive, we can offer more user-friendly functions in Ribasim python which create these tables in the background. This has earlier been proposed (but not yet implemented) here.