Make simplify work with geoms other than linestrings

[joto]

The simplify() function currently only works for linestrings.

• [X] It should also work for multilinestrings.
• [ ] It should also work for (multi)polygons.

The problem with simplify is

a) there are many different algorithms which have different properties (I believe only Douglas-Peucker is implemented in boost::geometry which we use) b) simplification can make geometries invalid, which is bad and we usually guarantee that we only import valid geometries. This is especially bad for polygons.

See #1663.

[joto]

See also https://github.com/kleunen/boost_geometry_simplify