GnuplotExportModule: Added possibility to export mean stress from the PrescribedGradient boundary condition.
PrescribedGradient: Modified updateCoefficientMatrix() so that sigma_xy and sigma_yx are computed separately instead of only computing the sum (sigma_xy+sigma_yx).
Merge branch 'master' of https://github.com/Micket/oofem
sparsenonlinsystemnm.h: Added forward declaration of SparseLinearSystemNM.