Fix plot_mode functions for 6 DoF model

[jguarato]

In discussion #1089, an issue was reported regarding plotting mode shapes for 6 DoF model. We need to investigate this further and implement a solution. Additionally, we still need to fix the plot_mode functions for axial and torsional modes.

[Pierre-zhu]

that's best! Thanks for your working!

[ross-bott]

Hi there!
I have marked this issue as stale because it has not had activity for 45 days.
Consider the following options:

• If the issue refers to a large task, break it in smaller issues that can be solved in
  less than 45 days;
• Label the issue as wontfix or wontfix for now and close it.