Revit_Toolkit: add support for curved panels

[pawelbaran]

Description:

Following merging #796, panel converts have been stabilized to support all types of planar elements. However, curved ones are not supported at all, which would be good to be covered in long term.

A neat (though heavy) way of getting the middle surface of a curved panel could be setting its justification to side (or top for floors/roofs), updating its type to have half of original thickness, then taking the newly created face.