standardize reduction factor

[visr]

          I made this plot of the smooth reduction factor:

how can I add it to equations.qmd? It has been added now.

Originally posted by @SouthEndMusic in #549 (comment)

Bart made a nicer reduction factor. We should probably use this one for all the reduction_factors, which is now generally a linear reduction. Most are hardcoded to work until 10m3, for Manning it is 10cm waterdepth. Let's see if we can share this and/or make it configurable.

[evetion]

Probably any sigmoid function will work?

[visr]

Probably. Sigmoid looks more expensive than this, and only approaches the right value, so it is not smooth at the threshold.

[visr]

Sorry I should've checked #549 better first, it already moves all reduction factors over.