testsuite::test_lambertw - check if lambertw function worksSynopsis:
(test_lambertw) run -> some known relationsDescription:
The script tests whether the Lambert W-function  provided by the
GNU Scientific Library (GSL)  fulfills some known relations .
In the absence of the GSL NEST uses a simple iterative scheme to
compute the Lambert-W function. In this case we apply less strict
criteria for the required accuracy.
The relationships tested are:
(1) the principal branch crosses the origin
(2) at -1/e both branches meet athe value W=-1
(3) the principal branch fulfills the "golden ratio" of exponentials
(4) the non-principal branch yields the same result as we find by
bisectioning for the problem given as an example in the documentation
of LambertWm1 with realistic parameters.
 Corless, R. M., Gonnet, G. H., Hare, D. E. G., Jeffrey, D. J., & Knuth, D. E.
(1996). On the lambert w function. Advances in Computational Mathematics
 Galassi, M., Davies, J., Theiler, J., Gough, B., Jungman, G., Booth, M.,
& Rossi, F. (2006). GNU Scientific Library Reference Manual (2nd Ed.).
Network Theory Limited.
 Weisstein, E. W. (1999). CRC Concise Encyclopedia of Mathematics.
"Lambert W-Function" Boca Raton, London, New York, Washington D.C.: CRC Press.
Author: July 2009, Diesmann