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.