Plot warping function only over section extents

[robbievanleeuwen]

@TLCFEM - I made a few change to the warping function plot so that it only displays over the extent of the section. Do you think this is a better implementation? Consider this example:

from sectionproperties.pre.library import channel_section
from sectionproperties.analysis import Section
geom = channel_section(200, 100, 16, 8, 15, 8)
geom.create_mesh(3)
sec = Section(geom)
sec.calculate_frame_properties()
sec.plot_warping_function()

With the current implementation you get this:

This PR gives:

I feel like this is a better visualisation, however would you ever want to visualise the entire field, even outside the section extents?

[TLCFEM]

Yes, this is for sure the more correct solution. I was not thoughtful and just wanted to plot something to show the difference.