using `include <NopSCADlib/lib.scad>` in main file fails in `screw_boss_diameter`:
`WARNING: Ignoring unknown function 'nut_radius', in file ../../../sw/OpenSCAD/libraries/NopSCADlib/vitamins/screw.scad, line 41.`
Maybe my library usage is wrong, I did not investigate further ...
Moved screw_insert() from inserts.scad to screw.scad.
Changed most includes of inserts.scad to use insert.scad.
Removed linear_bearings, iecs, rockers and microswitches from lib.scad as
already included by other vitamins.