fischertechnik compatible flex rail (Flexschiene) 155mm

This is (mainly) a 155mm long version of the fischertechnik flex rail.

When combining fischertechnik and Gravitrax (using this: https://www.thingiverse.com/thing:3169727 ), it turned out that the 180mm original rails were to long.

The knobs might be glued into the rail, but for us it worked as it is and without glue the knobs could easily be exchanged when broken.