Aircraft level Hub definitions (#1363)

* Aircraft level Hub definitions

* Add ability to define a hub for an aircraft apart from its subfleet.

* Update csv import/export capability for hub_id field

* Fix importertest source csv

* Fix source csv for updating too

* Update aircraft_empty_cols.csv

Co-authored-by: Nabeel S <nabeelio@users.noreply.github.com>

app/Services/ImportExport/AircraftImporter.php

@@ -25,6 +25,7 @@ class AircraftImporter extends ImportExport
         'subfleet'     => 'required',
         'iata'         => 'nullable',
         'icao'         => 'nullable',
+        'hub_id'       => 'nullable',
         'airport_id'   => 'nullable',
         'name'         => 'required',
         'registration' => 'required',
@@ -53,7 +54,7 @@ class AircraftImporter extends ImportExport
     }
 
     /**
-     * Import a flight, parse out the different rows
+     * Import an aircraft, parse out the different rows
      *
      * @param array $row
      * @param int   $index