implemented more robust handling of graph file

[rmm-fcul]

• resolves #16 .
• 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.