Replace the coefficient of abs in the lookup table of meijerint.

[jksuom]

The denominator of the coefficient pi/(cos(pi*a/2)*gamma(a))
becomes indeterminate for negative odd integers a where cos(pi*a/2)
has a zero and gamma(a) has a pole. There is no code for computing
the limit at such argument values. This PR replaces the coefficient
with 2*sin(pi*a/2)*gamma(1 - a), which is well defined for re(a) < 1,
since the only poles of gamma occur at nonpositive integers.

The equivalence of the two forms can be derived from the standard
formulas

gamma(a)*gamma(1 - a) = pi/sin(pi*a)

and

sin(pi*a) = 2*sin(pi*a/2)*cos(pi*a/2).

[smichr]

I double checked the derivation and it looks fine. Code changes look fine, too. I don't know a lot about meijerint, however, so will defer to someone else for any other comments. Lacking any, I am +1 to commit in 24 hrs.

[jksuom]

Thanks, @smichr !