Improve `erf` function for complex values

This call:
```java
    System.out.println(org.hipparchus.special.Erf.erf(new Complex(1.0, 1.5)));
```

prints out:
`(NaN, NaN)`

but should print
`(1.8468330146085414,-0.7013604642514801)`

See wolframalpha:
- https://www.wolframalpha.com/input?i=Erf%281.0%2B1.5*I%29