Moment interaction diagrams should terminate at M=0

[robbievanleeuwen]

Actions:

• Terminate moment interaction diagrams at M=0 so that point_in_diagram() works as intended. Currently for asymmetric diagrams it is possible to have a point reported as outside the diagram when it should strictly be in the diagram (conservative). However, there should currently be no unconservative reporting.
• Give (bring back) the option of plotting the negative diagram? (may assist with the above)
• More efficient max_comp computation. We don't need to compute above max_comp.

See discussion here and here.

Originally posted by @Agent6-6-6 in #66 (comment)