Added workaround for F_GB13 duplicates

Added workaround for F_GB13 duplicates

Under some conditions, identical reference bond lengths could lead to
different computed 1-3 lengths (using icc 11 and double precision, at
least). These then failed the memcmp() test that removes duplicate
parameter lists, which led to near-duplicates in the
function-parameters lists. In theory, we shouldn't test for
floating-point equality with memcmp(), but in theory the test should
work, too!

Instead of memcmp(), for F_GB13 only, tests with
gmx_within_tol(a,b,1e-6) are now used. That should be an acceptable
compromise between expected function, numerical accuracy and grompp
execution time.

IssueID #699