Add denisfy tool

[Notgnoshi]

Originally I thought this could be added to the simplification tool from #6. But now I think it should be its own tool, because I still have unanswered questions about how to best simplify geometries.

Two densification options:

• Fill N points; will leave longer gaps the longer the lines are. But that's maybe fine
• Fill ratio points; for each segment, determine how many points to fill based on its length
• Point spacing; variant of the ratio, just a different phrasing