fix modifiers: root is "!", disable is "*"

Hi, I am currently testing SolidPy and I find it very usable so far.
Your implementation seems to be much better than SolidPython, mainly because you are using methods, so the most important thing is in front (the geometric object) and modifications follow, which is nice.

I added three convenient unary operators:
  ~Obj -> Obj.debug = True
  -Obj -> Obj.disable = True
  +Obj -> Obj.background = True
These can be used as quick as the openscad modifiers.

While testing these I noticed that the modifiers root and disable were interchanged when generating openscad code, root should be ! and disable should be *