Fix head and clearance for the cycloid gear

[jbaehr]

Head and clearance values have been applied only once, but as we're dealing with a diameter, not a radius, they are needed twice. (The involute gear does not suffer from this bug)

Note that master suffers from this bug, too, but as this change is breaking, backporting needs careful consideration.

[jbaehr]

@looooo what do you think, should we backport this fix to master, too?

• on the one hand, without a fix only half of the specified clearance is actually applied. Which is bad and wrong.
• on the other, with a fix the actual clearance would double. It would be the specified value but under some circumstances this could break existing models:
  • if the clearance has been manually tweaked and now the dedendum becomes larger than the diameter of the hypocycloid's rolling circle, recomputation will fail (as of now, the code does not deal with this)
  • if other features are modelled very close to the gear's root circle, they could break through

Both cases are quite easy to fix, though: just divide the clearance value by two. But: you have to know!

@jacprog, you did a lot of work using the cycloid gears for your 3D printed clocks. What do you think, as a user?

[looooo]

@jbaehr I guess we can also apply the fix on master. But I am not sure how much sense this makes. In my mind it is ok to keep the commit in the develop branch only. I am not yet sure when to make a new release, but I guess this will happen in the spring. So best to concentrate on the develop branch. Btw. thanks for your work!!!