adding sign() to Giac parser

git-svn-id: http://dev.geogebra.org/svn/trunk/geogebra@30184 23ce0884-8a58-47d3-bc5c-ddf1cd5b9f9e
#3000: use the symbolic GIAC result if there is division by variable, avoid error message if something goes wrong during compute

git-svn-id: http://dev.geogebra.org/svn/trunk/geogebra@30185 23ce0884-8a58-47d3-bc5c-ddf1cd5b9f9e
#3857: some more refactoring

git-svn-id: http://dev.geogebra.org/svn/trunk/geogebra@30188 23ce0884-8a58-47d3-bc5c-ddf1cd5b9f9e
fix for derivative of If[x < x(A), x²,x³]

git-svn-id: http://dev.geogebra.org/svn/trunk/geogebra@30190 23ce0884-8a58-47d3-bc5c-ddf1cd5b9f9e