D tilde issues

[cristipurdel]

Would be nice to have this in the enginnering constants to make it crystal clear that in case of non-symmetric laminates the D tilde has smaller values.

Instead of not running the D tilde approximation at all, I would convert all values to absolute values and perform the analysis (could add a warning that the values were negative but positive values were considered).

In case of non-symetrical layup, the non-dimensional parameters are based on HSB not valid ?!? HSB 45910-01A says "The shell can be made of homogenous anisotropic material, but with symmetric lay-ups"

[cristipurdel]

implemented as of beta 3.0.0.0