tested with all the cases provided in the dialog on issue #16, plus the newly elected method to explicitly declare absence of nbg file (unilaterally proposed as None or null)
as noted below, no warnings are issued. If anyone objects, please suggest which cases require warnings, and their severity.
if a graph (nbg) file is not defined, no neighbours created for any
casu (since no specification)
if the nbg is defined as None, none, Null, null, NULL,, no neighbours
created for any casu (since no specification)
if a given layer has no subgraph defined, no neighbours defined for
those casus
if any casu in any layer is not in the relevant subgraph, no
neighbours are defined
finally, if there is a casu in the right subgraph and it has any
out_neighbours, these will be introduced in the rtc file.
NOTE: no warnings are issued for any of these cases.