binary_hard_sphere fails for small qr

[pkienzle]

The binary hard sphere fails for small values of qr, even in double precision:

sascomp -double binary_hard_sphere 1 0 radius_sm=10

The code uses expressions such as (sin(yy) - x cos(x))/x^3 and 2(cos(ay) - 1)/ay, though not directly. Rearranging the code so that these expressions are replaced by their stable equivalents may be enough.

[pkienzle]

Note that the binary_hard_sphere code gives slightly different values if large and small spheres and volume fractions are swapped, but the equations are symmetric so it shouldn't matter which sphere is labelled large and which is labelled small.