Problem::is2D() or similar needed

Now, in the code, in order to know if we deal with a 2D problem I use Problem::getFrameStructureSize() == 3 is used. However, a 2D POV frame structure (not implemented for the moment) wouldn't pass the condition...

Apart from the frame structure size, set when creating the problem, problem should have a boolean that specifies if 2D representation is used, for example, with the getter bool Problem::is2D().

Do you agree?