Create substance/concentration input tables

[evetion]

For the boundaries, and set a similar table for the state of a basin (given it also has boundaries).

After a long discussion with Martijn, we ended up with a separate table type that has
id, time, substance, concentration columns. In #1111 I'm still using a concentration field on the existing static/time tables for the boundary nodes.

This data is all used for Delwaq input when coupling, but could also be used in the future for our own tracer/salt calculations.

[Huite]

Conceptually, how would you make a distinction between drainage/infiltration concentrations versus precipitation? Or evaporation, which shouldn't subtract solutes (in general).

For what it's worth in terms of inspiration: MODFLOW 6 has a flow model and a transport model.
The flow model has a boundary, e.g. recharge: https://modflow6.readthedocs.io/en/latest/_mf6io/gwf-rch.html
In case of a linked transport model, the flow boundary has an auxiliary array defined with the concentration.
(A specific source can also be added separately, which does not add flow but does add solute.)

From the perspective of the user, solute concentrations are generally conceptually part of the boundary condition just as much as the flow (a hydrologist would prefer to think of the physical features first and foremost). In imod-python, we've chosen to add the concentration as an optional argument to the flow boundaries: https://deltares.github.io/imod-python/api/generated/mf6/imod.mf6.Recharge.html
This also makes it less likely that you forget to define concentrations for a boundary.

[evetion]

Done in #1267