Added documentation for Pressure_BC

web/content/docs/benchmarks/small-deformations/Pressure_BC/Pressure_BC.md

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+author = "Francesco Parisio"
+title = "Pressure boundary conditions"
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+
+##Problem description
+
+Five different benchmarks are reported in which pressure boundary conditions are tested: an axisymmetric elastic cylinder, a plain strain elastic cylinder, an axisymmetric elastic sphere, a tri-dimensional elastic sphere and an axisymmetric elasto-plastic sphere.
+See [this PDF](../pressure_bc.pdf) for detailed problem description.
+
+##Results and evaluation
+
+Plain strain elastic cylinder comparison between numerical and analytical results:
+{{< img src="../pipe_plane_strain.pdf" >}}
+
+Axisymmetric elastic cylinder comparison between numerical and analytical results.
+{{< img src="../pipe_axisymmetric.pdf" >}}
+
+Axisymmetric elastic sphere comparison between numerical and analytical results:
+{{< img src="../sphere_axisymmetric.pdf" >}}
+
+Tri-dimensional elastic sphere comparison between numerical and analytical results:
+{{< img src="../sphere_3d.pdf" >}}
+
+Axisymmetric plastic sphere comparison between numerical and analytical results:
+{{< img src="../sphere_axisymmetric_pl.pdf" >}}
+