[SofaGeneralLoader] Compute subElement by default for Gmsh format

[epernod]

This bug just pop up again in #967 and was referenced in issue #631
Several Gmsh file have non coherent topology information, for example Triangulation with edges on the border only. This mess up information feeding the topology container.

If this behavior is wanted, use <MeshGmshLoader createSubelements="0" /> and be sure to link the different topology buffer data to the appropriate components.
For example:

<MeshGmshLoader name="loader" createSubelements="0" />

<Node name="triangulation" >
   <TriangleSetTopologyContainer name="tri_topo" triangles="@../loader.triangles" />
</Node>

<Node name="border" >
   <TriangleSetTopologyContainer name="edge_topo" edges="@../loader.edges" />
</Node>

fixes #631

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This PR:

• builds with SUCCESS for all platforms on the CI.
• does not generate new warnings.
• does not generate new unit test failures.
• does not generate new scene test failures.
• does not break API compatibility.
• is more than 1 week old (or has fast-merge label).

Reviewers will merge only if all these checks are true.

[epernod]

[ci-build][with-all-tests]