Feat: add the base plane angle unit for all plane angle units

src/UnitConverter/Unit/PlaneAngle/PlaneAngleUnit.php

@@ -0,0 +1,34 @@
+<?php
+
+/**
+ * This file is part of the jordanbrauer/unit-converter PHP package.
+ *
+ * @copyright 2017 Jordan Brauer <[email protected]>
+ * @license MIT
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+declare(strict_types = 1);
+
+namespace UnitConverter\Unit\PlaneAngle;
+
+use UnitConverter\Measure;
+use UnitConverter\Unit\{ AbstractUnit, UnitInterface };
+
+/**
+ * Plane Angle unit base data class. Any new plane angle units should
+ * extend this class and implement their name, symbol and units;
+ * overriding the $unitOf and $base properties only if necessary.
+ *
+ * @version 1.0.0
+ * @since 1.0.0
+ * @author Jordan Brauer <[email protected]>
+ */
+abstract class PlaneAngleUnit extends AbstractUnit
+{
+  protected $unitOf = Measure::PLANE_ANGLE;
+
+  protected $base = Degree::class;
+}