How to apply face pressures

Posietive and/or negative face pressure are added using a condition called Face3D, for displacement as unknown, or FaceVel3D, for velocity as unknown. This condition uses two variables called NEGATIVE_FACE_PRESSURE and POSITIVE_FACE_PRESSURE to read assigned pressure values on the nodes of the condition and therefore it is necessaey to add them as varibles in the python file. Face3D and FaceVel3D,as linear triangular conditin, could be fined in custom_conditions of structural application.