FloatArray: implemented append and push_back functions.
Continued implementation of PrescribedGradientBCWeak.
PrescribedGradientBCWeak: Fixed some errors.
PrescribedGradientBCWeak: Continued implementation of weak periodic BCs.
PrescribedGradientBCWeak: Fixed some errors.
PrescribedGradientBCWeak: implemented periodicity constraint in corners
and made various improvements.
PrescribedGradientBCWeak: Assemble diagonal entries even though they are
zero, to make iterative PETSc solvers happy. Also some clean-up.
Changed implementation of PrescribedGradientBCNeumann so that it is now
treated as a special case of weak periodic BCs. This reduces duplication
of code.
PrescribedGradientBCWeak: fixed treatment of some special cases. Also
improved crack output in GnuplotExportModule.
XFEM: Started to implement composite vtkxml visualization for
PlaneStress2dXfem.
XFEM: Continued with visualization for PlaneStress2dXfem. Also made some
modifications in evaluation of branch functions.
Merge branch 'master' of https://github.com/Micket/oofem

Conflicts:
	src/oofemlib/prescribedgradientbcneumann.C
	src/oofemlib/prescribedgradientbcneumann.h
	src/oofemlib/prescribedgradientbcweak.h
Fixed some merge problems.
Geometry: corrected computation of tangential signed distance for
PolygonLine.
Delaunay: implemented separate treatment for rectangles.
XFEM: Modified computation branch functions and fixed some tolerances.
PrescribedGradientBCWeak: Improved construction of traction mesh for
non-periodic meshes.
FEI2dTrLin: Corrected global2local for points outside the triangle.
Extended Gnuplot visualization.
More Gnuplot export.