warning for negative radius

[sawickib]

I suggest to add warning if user choose node with negative radius.

When I was starting with agros2d, I had problems related with drawing geometry for axis-symmetrical cases. You have to avoid nodes with negative radius. So, for example, if you draw full circle, you will actually get two spheres, so the solution will be doubled.

[karban]

Partially done (add node or label using dialog or mouse click).