Bug fix: when no analytic derivation and change in x very small.

Manual cherry pick from commit:afc2454913340799206b6fb8c8305d0b9afc645a
-) fixed bug introduced in commit 66527df;
-) estimation options for data set must be given also when ppost=1 (fix in testsuite);

Manual cherry pick from commit:ab5b922e462471bf6daee611d16465719c5f5c7d
Merge pull request #383 from rattoma/4.3

4.3: cherry picked fixes to mode_compute = 5 and gsa