Easier to use function for grouped line merge? #2508

In PR #2482 there was a longer discussion for doing a "grouped linemerge" that can be useful for things like joining highways, railways, waterways etc. into longer line strings.

As a result of that PR we merged a flex configuration with an example implementation pure SQL. This can work as a template for others to extend, but it is not that easy to understand and use. For the moment it is unclear how to improve on this but maybe this is something that can be built into Themepark later on.