Incorporate SmoothInterpolation

[SouthEndMusic]

Fixes #1482
Fixes #566

• If we actually use this, it would probably make sense to give you admin rights on SouthEndMusic/SmoothInterpolation.jl (github.com)
• Whether it is actually useful to use smoothing of the linear interpolations remains to be seen. Currently the implementation uses a sampled SmoothedLinearInterpolation which is a LinearInterpolation. The only new interpolation type used in the simulation is LinearInterpolationIntInv, which is a much more concise implementation of what was in get_area_and_level
• One takeaway of this PR regardless of the use of SmoothedLinearInterpolation is that we can get rid of quite some utility code, also partly because of making more use of derivatives and integrals via DataInterpolations.jl

[SouthEndMusic]

I smelled something fishy when I found out that on this branch the AGV model takes 3-4x longer than on main. No tests are failing, and noting remarkable shows up in the profile. So I investigated a bit, and I found that the profile for certain basins behaves weird in the old code:

I haven't figured out what triggers this yet, but apparently it doesn't happen in our test models, at least not where tests depend on it. I'm pretty sure the new code (in SmoothInterpolation) is more correct and robust because it is tested more elaborately.

[visr]

Great that we can get rid of so much utility code, and have a more solid basis to work on.