[web] GMSH2OGS documentation for -e and -b.

web/content/docs/tools/fileio/GMSH2OGS/index.md

++++
+date = "2021-03-06T00:00:00+01:00"
+title = "GMSH2OGS"
+author = "Dmitri Naumov"
+
+[menu]
+  [menu.tools]
+    parent = "Data Import/Export"
++++
+
+## Introduction
+
+Meshes generated with Gmsh can be converted to OGS meshes, in particular to VTU
+file format.
+Currently, the only supported Gmsh format is 2.1, which can be specified for
+example with the command line flag `-format msh2` when executing `gmsh`.
+
+## Usage
+
+Gmsh writes all elements (unless specified otherwise) including the lower
+dimensional elements, usually model boundaries.
+Whereas for OGS we separate the so called "bulk" mesh and the boundary (or
+subdomain) meshes.
+`GMSH2OGS` options `-e` (`--exclude-lines`) and `-b` (`--boundaries`) control
+whether the line elements are removed or written in separate files.
+
+## Example
+
+A 2D-mesh with two physical groups (MaterialIDs) was generated with Gmsh.
+It also includes the outer boundary and inner boundary line elements.
+
+### Input files and results
+
+All files are stored in
+[Tests/Data/MeshLib/](https://gitlab.opengeosys.org/ogs/ogs/-/blob/master/Tests/Data/MeshLib):
+ - the Gmsh generated mesh
+[A2-gmsh.msh](https://gitlab.opengeosys.org/ogs/ogs/-/blob/master/Tests/Data/MeshLib/A2-gmsh.msh),
+ - and the corresponding result files
+[A2.vtu](https://gitlab.opengeosys.org/ogs/ogs/-/blob/master/Tests/Data/MeshLib/A2.vtu),
+and the boundary meshes
+[A2_0.vtu](https://gitlab.opengeosys.org/ogs/ogs/-/blob/master/Tests/Data/MeshLib/A2_0.vtu) to
+[A2_7.vtu](https://gitlab.opengeosys.org/ogs/ogs/-/blob/master/Tests/Data/MeshLib/A2_7.vtu).
+
+### Running GMSH2OGS
+
+`GMSH2OGS` creates different meshes from the same input depending on the `-e`
+and `-b` flags; the results are shown (z-translated) in the figure below.
+
+```bash
+GMSH2OGS -i A2-gmsh.msh -o A2.vtu
+```
+
+ - Conversion without any additional flags yields a single VTU file with
+   outer and inner boundaries (line elements) included, shown in the very
+   bottom.
+ - Adding only a `-e` flag results in a single VTU file, now without the line
+   elements.
+ - Adding only a `-b` flag gives a single VTU file as in the first case *and*
+   additional eight files, each containing a line-element mesh corresponding to
+   the physical groups;
+   these are the white lines in the figure, shown again z-translated.
+ - Finally specifying both flags (`-e` and `-b`) produces a single VTU file
+   without the line elements and additional eight boundary (subdomain) files.
+
+![GMSG2OGS-e-b](./extract_boundary.png#two-third "GMSH2OGS meshes for -e and -b
+command line flags.")
+