Adding FgradVisualize.m

[dagiroxforever7]

Added a function that helps with visualizing deformation gradients.

[Kevin-Mattheus-Moerman]

@dagiroxforever7 thanks for using GIBBON and for this pull request!

Some comments on your PR:

• Can you make the function more performant by pre-allocating arrays.
• Can you remove unused variables and the "double" computation of J in the example.
• Can you check if your function can make use of the quiverTriad function (apologies not documented very well yet)?
• Can you extend the demo to also feature an example where the deformation gradient tensor contains a significant rotational aspect?
• Once your example is finalised, can you run gpublish HELP_FgradVisualize? This should also create the relevant website documentation files for your new function.

Thanks again, let me know if you have questions.