net-sim: skip files missing on the card; /api/write accepts a full path

Live-testing against a UICC without the EPS files (no USIM service 85)
and without the Kc files showed the runner aborting when EPSLOCI/Kc were
absent.  write_binary/write_record now take an 'optional' flag: a missing
candidate file is logged as a skip step instead of failing the scenario
(store_epsnsc, real/dummy locations, churn and Kc writes).  Also allow
POST /api/write to select by 'path' like /api/select and /api/read (the
netsim live test needed it to restore the captured file values).

pysim_simple_server/server.py

@@ -3706,6 +3706,7 @@ class PysimHandler(BaseHTTPRequestHandler):
             parent_sel = body.get('parent_sel')
             parent_path = body.get('parent_path')
             allow_probe = bool(body.get('allow_probe'))
+            path = body.get('path')
             rs = app.rs
             if not rs:
                 self._send_json({'error': _err('no_card_state', lang)}, 503)
@@ -3714,8 +3715,11 @@ class PysimHandler(BaseHTTPRequestHandler):
             lchan = rs.lchan[0]
             cleanup = None
             try:
-                sel = fid if fid else name
-                _, cleanup = _select_with_parent(lchan, sel, parent_sel, app, parent_path, allow_probe)
+                if path:
+                    _, cleanup = _select_path(lchan, path, app)
+                else:
+                    sel = fid if fid else name
+                    _, cleanup = _select_with_parent(lchan, sel, parent_sel, app, parent_path, allow_probe)
                 ft = _get_file_type(lchan, lchan.selected_file)
                 is_record = ft in ('linear_fixed', 'cyclic')
                 if record_nr: