Simple change to the Formula class

Some formula writing would be easier if the Formula class in common.ulb had in its protected area

   complex m_c

and functions like

  func SetC(complex c)
    m_c = c
  endfunc
  complex func GetC( ) 
    return m_c
  endfunc

Including these changes would not break any current fractals and would allow simpler switching from Mandelbrot-type to Julia-type.

And of course there could be more complications, like arrays of values, and floats, and ints. Still none of this would break old fractals.

Good idea, I'm open to suggestions regarding updating the classes in common.ulb.

I'm happy to produce a new version. I've already made a simple version of such a formula, FormulaX in jlb.ulb. It also has the Mandelbrot/Julia switch built in. What else makes sense to put in, without getting too complicated?