Add ability to calculate properties for above and below the neutral/shear axis

[EngDM]

There are some pretty niche calculations required for monosymmetric beams that involve splitting the member at it's shear center and re-calculating centroids for the shape above and below this axis. It would be nice if there was a way to feed an x-y coordinate to the analysis call and it calculates properties based on this input, returning values for above-below, left-right of this specified coordinate. If it accepts an input of coordinate instead of just assuming shear center, then we could pass it the shear center OR a coordinate.

This could also be achieved by creating new sections for top and bottom, although I'm not sure how you'd go about splitting the sections.

[robbievanleeuwen]

You can already split a Geometry object either side of a line. This method returns Geometry objects either side of the line.