Milestone build default tinkerbots models

[stefaniapedrazzi]

The current version of the robot designer is very close to allow to build all the default models.
But there are a couple of issues with the slots that prevents it:

• missing slots definition for cubie_box_with_round_slots
• translation step is too big for the front face of the cubie_box_with_round_slots and cubie_box_with_cross_slots -> it is not possible to move the attached part precisely to the wanted position

Additionally:

• it should be possible to define multiple slot type for round female slots: they can be connect with both cross and round male parts

[fabienrohrer]

• translation step is too big for the front face of the cubie_box_with_round_slots and cubie_box_with_cross_slots -> it is not possible to move the attached part precisely to the wanted position

Just some indication: This is linked with #7. The constraints should be different for each slot rather that hard-coded generically.

[stefaniapedrazzi]

With the fixes in #23 and changing the default translation snap for leg slots from 0.02 to 0.01 (even if it is better to not hard code them) all the default models can be built.

[stefaniapedrazzi]

Last remaining issues fixed in #24.