Translations:Add FEM Equation Tutorial/3/en

The task can be split into five parts:
 * New equation type. This step must only be done if the equation doesn't exist in FreeCAD yet (as opposed to a equation that is already in FreeCAD but not supported by the target solver).
 * New equation object. Adding a concrete document object representing the elmer specific equation.
 * Extend solver object. Adding support for the new equation to the solver object of elmer.
 * Extend writer object. Extending the analysis export of elmer to support the new equation type.
 * Gui tool to create a equation. Access the new equation function through workbench Gui.