[Bug-gsl] gsl_sf_coupling_6j_e and gsl_sf_coupling_9j_e input validation

[Håkan Johansson]

The coupling of three spins requires the total sum to be integer.  This is 
not checked on gsl_sf_coupling_6j_e or gsl_sf_coupling_9j_e.

We discovered it when trying to evaluate the 9j symbol:

(1/2 1/2 1/2)
(1/2 1/2 1/2)
( 0   0   0 )

and got a non-zero result.  We believe that the recursive calculations of 
6j should have given the zero, but found also that lacking similar checks.

The attached patch also include some test cases.  The 6js have been 
cross-checked with Mathematica.

Best regards,
Håkan Johansson

coupling_6_9_halfint_check.diff
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