simplification using a %-like value

[mattijn]

Raised at the dependency page, urschrei/simplification#11.
But also noting this page: https://grasswiki.osgeo.org/wiki/V.generalize_tutorial

Douglas-Peucker Algorithm has very reasonable results, but it is very hard to find(guess) the right value of threshold. Moreover, it is also impossible to simplify each line to (for example) 40%. Exactly for such cases, v.generalize provides method=douglas_reduction. This algorithm is a modification of Douglas-Peucker Algorithm which takes another paratemer reduction which denotes (approximate) simplification of lines.

Which seems to come from here: https://github.com/giscan/Generalizer/tree/master