Ultimaker frame holds up to 4 cassettes (v.2)

This is version 2 of our work to equip the Ultimaker 3D printer with up to 4 exchangeable 'cassettes'. For v.1 and development rationale, see Thing 25836: http://www.thingiverse.com/thing:25836.

For this version, we decided to use 3D-printed parts alongside birchwood.

(Developed at Protospace Utrecht with help from Douwe van der Veen/appultaart