allow different algorithms for evaluating erf

[benjaminfjones]

In #1173 an mpmath numerical evaluator for erf was written by Douglas McNeil. This ticket proposes to save that code, incorporate it with the ability to pass an algorithm parameter to numerical_approx from #12289. We could support Pari, mpmath, and Maxima.

It would even be useful to see where Pari is faster.

[benjaminfjones]

add mpmath evaluation to erf

[kcrisman]

See comments at #11948 for why we probably want to do this or #1173 as soon as possible.

[jdemeyer]

This looks like a duplicate of #1173 to me.

[kcrisman]

This looks like a duplicate of #1173 to me.

I think that the point of this ticket was to do some of the things at #1173 more quickly, or using a different infrastructure, or something. Ben, what was the rationale exactly?

[kcrisman]

I think that we can repurpose this ticket to adding the algorithm keyword here. Some good examples from #1173 (most were added in #13001):

Verify we're returning the appropriate zero:: 

            sage: erf(0) 
            0 
            sage: erf(0.0) 
            0.000000000000000 
            sage: erf(RealField(100)(0)) 
            0.00000000000000000000000000000 

set(erf(45*10**i).n() for i in range(10)) 

sage: CC(erf(ComplexField(1000)(2+3j)))