[Bf-python] Final Ipo/IpoCurve API revisit

[Ken Hughes]

Stephen Swaney wrote:
> 
> In general, constants are preferable to strings for reasons we have
> already discussed.

OK, I'm trying to convert to the constant implementation.

> A couple thoughts:
> 
>  One solution to the string/constant problem is to support constants
>  for most IPOs and strings for Key/Shapes.  We can do this by
>  overloading the methods to take both types, something we already to
>  in other cases.

That's probably a good compromise.

> A side thought:
> 
>  For the different IPO types, it should be possible for the user to
>  tell what curves are available for that IPO type.

I already have an attribute which returns a list (using strings as of 
now) of valid curves.  It can be used as a reference or for iteration.

> Ken, would you like to put up a brief design document for your work
> on the wiki?

I would, but only if it's going to be useful/used.  I've been asking for 
feedback thru the mailing list since December and hav received very 
little.  If the wiki page is going to sit uncommented on for a few 
months then it's not worth it.

Ken