atan (Matlab function) Two-quadrant inverse tangent

## Matlab/Scilab equivalent

Matlab | Scilab
    atan
    atan

## Particular cases

When the input `x` is imaginary and `abs(x)>=1`, the real part of the result differs from Scilab vs Matlab or Octave:

| Scilab | Octave | Matlab
atan(-2i) | -π/2 - B.i | -π/2 - B.i | π/2 - B.i
atan(-i) | Nan - Inf | 0 - Inf | 0 - Inf
atan(i) | Nan + Inf | 0 + Inf | 0 + Inf
atan(2i) | π/2 + B.i | π/2 + B.i | -π/2 + B.i