[giac] make sure 2 instances can run at once (desktop)

git-svn-id: http://dev.geogebra.org/svn/trunk/geogebra@25373 23ce0884-8a58-47d3-bc5c-ddf1cd5b9f9e
[giac] make sure "return angles from inverse trig functions" works

git-svn-id: http://dev.geogebra.org/svn/trunk/geogebra@25374 23ce0884-8a58-47d3-bc5c-ddf1cd5b9f9e
[giac] implement atan2(y,x)

git-svn-id: http://dev.geogebra.org/svn/trunk/geogebra@25375 23ce0884-8a58-47d3-bc5c-ddf1cd5b9f9e