STRCUR for STRAIN command

[victorkemp]

Added the option of STRCUR or FIBER to the case control STRAIN command for consistency with MSC Nastran. FIBER is the existing behavior of Z1 and Z2 strains. STRCUR outputs membrane strain and curvature.

The default is now STRCUR so it changes the behavior of all decks that request shell strain without specifying either.

Applies to CTRIA3, CQUAD4 (MIN4T, MIN4 and MITC4+), and CQUAD8.

The relationship between the two forms is:

<strain at Z1> = <membrane strain> - <Z1> * <curvature>
<strain at Z2> = <membrane strain> - <Z2> * <curvature>

OP2 output is updated, including it's flag bit for not fiber, but not tested.

F06 block column headers are re-aligned to be consistent for all the permutations of stress/strain, fiber/strcur, and von Mises/maxs. The data itself is still formatted the same.

Outstanding issues:

• Derived quantities Smax, Smin, vom Mises/max shear are probably still being calculated wrong as with FIBER.
• Max and min summary rows won't make sense because they combine membrane strain and curvature.
• In MSC, STRCUR and FIBER can be set differently in different subcases. Mystran applies the same setting to all subcases.

I'll make a corresponding PR for the validation suite to add tests for this and update old tests that it breaks.