[giac] improvements to FitXXX

git-svn-id: http://dev.geogebra.org/svn/trunk/geogebra@24825 23ce0884-8a58-47d3-bc5c-ddf1cd5b9f9e
[giac] updated unit tests

git-svn-id: http://dev.geogebra.org/svn/trunk/geogebra@24826 23ce0884-8a58-47d3-bc5c-ddf1cd5b9f9e