Replace -x with fabs(x) due to odd breaking Linux behavior

[chinandrew]

Within the erf() function, my code was getting hanging on this statement

    if (x < 0.0) {
	return -erf(-x);
    }

due to -x not being successfully passed into the recursive call (i.e. it was still negative).

Both

    double a;
    if (x < 0.0) {
    a = -1.0*x;
	return -erf(a);
    }

and

    if (x < 0.0) {
	return -erf(fabs(x));
    }

work, as did changing the implementation back to the scipy.special one.

Also noting that the following did not work.

    if (x < 0.0) {
	return -erf(-1.0*x);
    }

This PR implements fabs() approach to fix the issue. It was chosen after discussion with @aki-nishimura for simplicity and to avoid any potential other issues with doing a -1 multiplication.

This problem occurred in the following environments:

1. Kubuntu 21.04, gcc version Ubuntu 10.3.0-1ubuntu1
2. Mint 20.1, gcc version Ubuntu 9.3.0-17ubuntu1~20.04 (Mint 20.1 is based on Ubuntu 20.04)

It worked fine for me on windows 10 and all OS's pass on Github Actions, where ubuntu-20.04 is currently the latest supported linux version.

Exact root cause of this issue has not been investigated.