ca_erf, ca_erfc, ca_erfi

Sets res to the error function $\operatorname{erf}(x)$, the complementary error function $\operatorname{erfc}(x) = 1 - \operatorname{erf}(x)$ or the imaginary error function $\operatorname{erfi}(x) = -i \operatorname{erf}(ix)$. Odd symmetry is used to normalise the argument, and the relations between the three functions at related arguments are added to the ideal of relations. For special values, $\operatorname{erf}(\pm \infty) = \pm 1$, $\operatorname{erfc}(+\infty) = 0$, $\operatorname{erfc}(-\infty) = 2$, $\operatorname{erf}(\pm i \infty) = \pm i \infty$, and unsigned infinity gives Undefined.