erfc is speeded for float scalar and simd horner is used
and in horner a macro NT2_HORNER_RAT is added to ease the computation of rationnal fractions P/Q
changing benches ranges for erfc
suppressing NT2_RAT in favor of NT2_HORNER_RAT
and adding a guarded test for infinity which was missing in the float case
after Mathias advise replacing rational fractions in erfc by polynomials with no loss
removed unused include + move includes around in erfc
Merge pull request #681 from jtlap/speed_erfc

Speed erfc