Save element parameters in its own database table (dont use JSON)

This makes it easier to query for specific parameters.

src/Entity/Parameters/MeasurementUnitParameter.php

@@ -0,0 +1,40 @@
+<?php
+/**
+ * This file is part of Part-DB (https://github.com/Part-DB/Part-DB-symfony).
+ *
+ * Copyright (C) 2019 - 2020 Jan Böhmer (https://github.com/jbtronics)
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as published
+ * by the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+namespace App\Entity\Parameters;
+
+use App\Entity\Parts\MeasurementUnit;
+use Doctrine\ORM\Mapping as ORM;
+
+/**
+ * @ORM\Entity()
+ * @package App\Entity\Parameters
+ */
+class MeasurementUnitParameter extends AbstractParameter
+{
+    public const ALLOWED_ELEMENT_CLASS = MeasurementUnit::class;
+
+    /**
+     * @var MeasurementUnit the element this para is associated with
+     * @ORM\ManyToOne(targetEntity="App\Entity\Parts\MeasurementUnit", inversedBy="parameters")
+     * @ORM\JoinColumn(name="element_id", referencedColumnName="id", nullable=false, onDelete="CASCADE").
+     */
+    protected $element;
+}