fixes #79

@Doyeux: you should use the helper function exporter for example

```
auto ex = exporter( _mesh=mesh, _geo=EXPORTER_GEOMETRY_STATIC);
ex->step(time)->setMesh(mesh);
ex->step(time)->add("u", u);
ex->save();
```

will make sure that the geometry is saved only once

```
auto ex = exporter( _mesh=mesh, _geo=EXPORTER_GEOMETRY_CHANGE_COORDS_ONLY);
ex->step(time)->setMesh(mesh);
ex->step(time)->add("u", u);
ex->save();
```

will add changes_coords_only in the .case file next to the model definition to
tell ensight/paraview that only the coordinates are changed (e.g after moving
the mesh in fsi). This is the default behaviour.

```
auto ex = exporter( _mesh=mesh, _geo=EXPORTER_GEOMETRY_CHANGE;
ex->step(time)->setMesh(mesh);
ex->step(time)->add("u", u);
ex->save();
```
Merge branch 'release/version-0.92' of https://github.com/feelpp/feelpp into release/version-0.92

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Merge branch 'release/version-0.92' of https://github.com/feelpp/feelpp into release/version-0.92

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# especially if it merges an updated upstream into a topic branch.
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