Fiber section invert_xaxis() for z-coordinate

[mhscott]

Adding call to invert_xaxis() in plot_fiber_section so that the section z-coordinate is positive to the right like in most documentation. Without inverting the axis, it can be confusing when building patches one at a time and seeing the patches "on the wrong side".

[sewkokot]

Thank you @mhscott! Indeed now it is consistent!