added rounding rules and hourly rates factor #112 (#128)

* added unit tests and documentation #112
* fixed DashboardController
* added CONTRIBUTION guidelines
* fixed license year

src/Timesheet/CalculatorInterface.php

@@ -0,0 +1,28 @@
+<?php
+
+/*
+ * This file is part of the Kimai time-tracking app.
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace App\Timesheet;
+
+use App\Entity\Timesheet;
+
+/**
+ * A calculator is called before a Timesheet entity will be updated.
+ * These classes will normally be used when calculating duration or rates.
+ */
+interface CalculatorInterface
+{
+    /**
+     * All necessary changes need to be applied on the given $record.
+     * The methods return value will not be evaluated.
+     *
+     * @param Timesheet $record
+     * @return void
+     */
+    public function calculate(Timesheet $record);
+}