Add tutorial notebook about the Constant element in TimML

[dbrakenhoff]

Discussed in https://github.com/mbakker7/timml/discussions/157

^{Originally posted by Huite February 23, 2024}
(After getting some more questions via email)

It is somewhat suprising to place a well, extracting water, and get positive isohypses. I think people (naively) expected a well to cause a drawdawn (relative to some zero). For the well, this works out to be a zero head at the well location, but this seems to end up confusing people a great deal. A single head well gives obviously weird results (-5.25e14), which is arguably less confusing.

Anyway, I'm wondering whether it doesn't make more sense to simply require a constant or a semi-confining top before solving a model, raising an error otherwise. Or is there any use case where you'd actually want "free-floating" elements?

[dbrakenhoff]

From @mbakker7 answer:

All in all, I don't think it should be required to add a Constant, although most modelers probably will add a Constant (if they simulate flow in a confined system). I think this is best explained with a good tutorial.