Construct Cartesian vector from spherical coordinates.

        alpha and delta must be in radians.

def from_spherical(cls, r=1.0, alpha=0.0, delta=0.0):
        """Construct Cartesian vector from spherical coordinates.

        alpha and delta must be in radians.
        """
        x = r * math.cos(delta) * math.cos(alpha)
        y = r * math.cos(delta) * math.sin(alpha)
        z = r * math.sin(delta)
        return cls(x=x, y=y, z=z)