Adding capability to write multiple failure values to f5 file

[timryanb]

• shell/beam f5 files will now have 5 unique failure fields ("failure0", "failure1", etc.)
• This is handled by new evalFailureFieldValue method in TACSConstitutive class
• This allows for multiple failure mode contours to be visualized in post-processing (as was requested in PR's Blade stiffened shell improvements #319 and Gaussian Process Buckling Constraints in Blade Stiffened Shell Constitutive Subclass #311)
• "failure0" defaults to whatever is given by evalFailure method (same value we've previously been writing out) and the rest default to 0 (This can be overridden at the sub-class level)
• Closes Add ability to write out multiple failure values in f5 #320

[A-CGray]

Tested this out and it works for the blade stiffened shell model

[A-CGray]

Are you happy for me to merge this @timryanb or do you want to wait for another review?

[timryanb]

I'm fine with that @A-CGray