Implement Grid.boundary_node_connectivity

[philipc2]

From the UGRID conventions:

boundary_node_connectivity pointing to an nBoundaryEdges X 2 index variable identifying for every edge of each boundary the two nodes that it connects. Again the indexing convention of boundary_node_connectivity should be specified using the start_index attribute to the index variable (i.e. Mesh2_boundary_nodes) and 0-based indexing is the default. Although constructed of edges, boundaries represent a different quantity than general edge data and thus the boundary_node_connectivity attribute may be specified independent of edge_node_connectivity. Information about the nature of each boundary edge (e.g. open/closed, land/water, grouping, etc.) may optionally be stored in ancillary boundary-type variables of size nBoundaryEdges X 1