Simplified Duplo Track Improved

I liked the simplified Duplo Track design, as it's very easy to print, even if bed is not sticking well. But I improved a few things I had issues with:

* Improved interlocking mechanism so it's easier to slide things into one another.
* Made rail surface wider, and distance narrower to allow more play for the train.
* Added grip profile, as I had issues with not enough traction while pulling a few wagons.
* Removed middle piece in straight track, as it seems strong enough as is
* Added half-length pieces for both straight and curved tracks