Plots the variation of stress as the coordinate rotates
2d case
Imagine a cube in space, and a source of stress in the x direction. Initially there is no stress in any other direction, and there is no shear stress. However, as we rotate the cube along the z axis, the stress splits into an x and a y component, plus a shear stress (xy shear = yx shear) component. These three stresses forms the three axes in 2DMohrsCircle.py
3D case
The same situation as above, but this time the x,y,z principal stresses are plotted (and ends up as a plane) as we rotate the cube in all directions using a sphere covering curve.
The xy(=yx), yz(=zy), zx(=xz) stress forms a sphere instead.
