Translations:Add FEM Equation Tutorial/3/en

The task can be split into four parts. The first step is to make the FEM workbench aware of a new type of equation. This step must only be done if the equation doesn't exist in FreeCAD jet (as opposed to a equation that is already in FreeCAD but not supported by the target solver). The secound step is to add a concrete document object representing the elmer specific equation. The third step is to add support for the new equation to the solver object of elmer. After that the analysis export of elmer must be extended to support the new equation type.