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/Database/migrations/2021_11_27_132418_add_hub_to_aircraft.php

@@ -0,0 +1,15 @@
+<?php
+
+use App\Contracts\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+class AddHubToAircraft extends Migration
+{
+    public function up()
+    {
+        Schema::table('aircraft', function (Blueprint $table) {
+            $table->string('hub_id', 5)->nullable()->after('airport_id');
+        });
+    }
+}