Add erf function

[dpsanders]

I believe it is implemented:

http://fredrikj.net/blog/2016/03/taking-the-error-out-of-the-error-function/

[JeffreySarnoff]

It is -- one of the complex funcs.
I am rewriting the package in part to support the Complex algorithms (which are used to get Real results sometimes), and to update the overall approach.
I will let you know when that is useable.

[JeffreySarnoff]

[also I will see what I can do for this particular case with ArbFloats]
as I expected, using it requires supporting the complex type --
If the rewrite takes too much longer, I may do that here.