[DEM] How custom submodelparts should look like

[farrufat-cimne]

#697
Assigning a group of particles (nodes) into custom-dem condition that are already defined as "Parts" is written as:

Begin Elements SphericParticle3D// GUI group identifier: custom
End Elements

Begin NodalData RADIUS // GUI group identifier: custom
End NodalData

Begin SubModelPart DEM-CustomSmp_custom // Group custom // Subtree DEM-CustomSmp
    Begin SubModelPartNodes
         1186
         1215
         1240
         1242
         1329
    End SubModelPartNodes
    Begin SubModelPartElements
    End SubModelPartElements
    Begin SubModelPartConditions
    End SubModelPartConditions
End SubModelPart

Is this the correct format required?
Does it require to rewrite the elements associated to the nodes in the submp?

When assigned to elements instead of nodes, it does not write anything on the mdpa:

Problem Writing MDPA block:
Element SphericPartDEMElement3D Linear (2 nodes) not available for line entities on group DEM-CustomSmp Auto1
End problems

[farrufat-cimne]

@jginternational @maceligueta