Technic pin friction primitives

[Owen Burgoyne]

I guess that's decided then!

From looking at the "Technic connector primitives" on the Primitives Reference page I see about four main friction primitives:

• short with rounded ends
  
• long with rounded ends
  
• short with square ends (confric3.dat)
  
• long with square ends (confric3.dat)
  

As the last two are currently specific to one part, I'll probably just create the first two for the time being.

There are also the thin frictionless ribs used on connect7.dat, but again these are specific to one part.