Explicitly consider "system margins" instead of stashing them to transponder modes

Since 30234f913c, the code just added the
"system margins" to each transponder's minimal required OSNR. That's
simple and straightforward, but I think that mutating "equipment
library" in response to a "global simulation parameter" is not really
the right way to go.

Make this explicit; always check the resulting OSNR against the
transponder's minimum plus the margin.

I got into this when checking the fixups that are performed within the
JSON loader. I don't think that the JSON loader is an appropriate place
for this.

Change-Id: Ic8f8385cbbb29dc0d769462f17caad47c2b36940

gnpy/tools/cli_examples.py

@@ -410,7 +410,8 @@ def path_requests_run(args=None):
                         f'-', f'{rqs[i].blocking_reason}']
         except AttributeError:
             line = [f'{rqs[i].request_id}', f' {rqs[i].source} to {rqs[i].destination} : ', psnrb,
-                    psnr, f'{rqs[i].OSNR}', f'{rqs[i].tsp_mode}', f'{round(rqs[i].path_bandwidth * 1e-9,2)}',
+                    psnr, f'{rqs[i].OSNR + equipment["SI"]["default"].sys_margins}',
+                    f'{rqs[i].tsp_mode}', f'{round(rqs[i].path_bandwidth * 1e-9,2)}',
                     f'{ceil(rqs[i].path_bandwidth / rqs[i].bit_rate) }', f'({rqs[i].N},{rqs[i].M})']
         data.append(line)