Full support of Nastran PBAR entry

[ThomasAbballe]

In Nastran, PBAR defines the properties of a simple beam element (CBAR entry) (MD Nastran 2006 Quick Reference Guide p2092).

The most common uses of this keyword is recognized and correctly translated by VEGA. However, if the user specifies some stress recovery coefficients, VEGA dismiss them, putting a warning:
"PBAR Stress coefficients not implemented. Warning: Line: nnn in file: foo.dat"

We should complete the support of this keyword by VEGA (and ensure its correct translation into Systus, but that's just me;)