Decide whether to modify the Mesh printout to group optional connectivities by their source element

[lbdreyer]

Should we modify the new Mesh printout, removing the "optional connectivities" section, and instead place them in their relevent source element section, and add an extra section for boundaries. e.g. something like (taking @pp-mo's example from #4499)

    topology_dimension: 2
    node
        node_dimension: 'nMesh2d_half_levels_node'
        node coordinates
            <AuxCoord: longitude / (degrees)  <lazy>  shape(55298,)>
            <AuxCoord: latitude / (degrees)  <lazy>  shape(55298,)>
    edge
        edge_dimension: 'nMesh2d_half_levels_edge'
        edge_node_connectivity: <Connectivity: Maps every edge/link to two nodes that it connects. / (unknown)  <lazy>  shape(110592, 2)>
        edge coordinates
            <AuxCoord: longitude / (degrees)  <lazy>  shape(110592,)>
            <AuxCoord: latitude / (degrees)  <lazy>  shape(110592,)>
        optional connectivities:
            edge_face_connectivity: <Connectivity: neighbor faces for edges / (unknown)  <lazy>  shape(110592, 2)>
    face
        face_dimension: 'nMesh2d_half_levels_face'
        face_node_connectivity: <Connectivity: Maps every face to its corner nodes. / (unknown)  <lazy>  shape(55296, 4)>
        face coordinates
            <AuxCoord: longitude / (degrees)  <lazy>  shape(55296,)>
            <AuxCoord: latitude / (degrees)  <lazy>  shape(55296,)>
        optional connectivities:
            face_face_connectivity: <Connectivity: Indicates which other faces neighbor each face / (no_unit)  <lazy>  shape(55296, 4)>
            face_edge_connectivity: <Connectivity: Maps every face to its edges. / (unknown)  <lazy>  shape(55296, 4)>
    boundary
        boundary_dimension: 'nMesh2d_half_levels_boundary'
        boundary_node_connectivity: <Connectivity: unknown / (unknown)  <lazy>  shape(1417, 2)>
    long_name: 'Topology data of 2D unstructured mesh'
    var_name: 'Mesh2d_half_levels'

This was an outstanding decision from #4499. See this comment for more details.