improved invoices with new renderer (#306)

* added docx renderer and demo template
* added csv renderer and demo template
* added xlsx renderer and demo template
* added ods renderer and demo template
* added user calculator
* added invoice documentation

src/Invoice/Calculator/AbstractCalculator.php

@@ -0,0 +1,111 @@
+<?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\Invoice\Calculator;
+
+use App\Entity\Timesheet;
+use App\Model\InvoiceModel;
+
+abstract class AbstractCalculator
+{
+    /**
+     * @var string
+     */
+    protected $currency;
+
+    /**
+     * @var InvoiceModel
+     */
+    protected $model;
+
+    /**
+     * @return Timesheet[]
+     */
+    abstract public function getEntries();
+
+    /**
+     * @return string
+     */
+    abstract public function getId(): string;
+
+    /**
+     * @param InvoiceModel $model
+     */
+    public function setModel(InvoiceModel $model)
+    {
+        $this->model = $model;
+    }
+
+    /**
+     * @return float
+     */
+    public function getSubtotal(): float
+    {
+        $amount = 0;
+        foreach ($this->model->getEntries() as $entry) {
+            $amount += $entry->getRate();
+        }
+
+        return round($amount, 2);
+    }
+
+    /**
+     * @return float
+     */
+    public function getVat(): ?float
+    {
+        return $this->model->getTemplate()->getVat();
+    }
+
+    /**
+     * @return float
+     */
+    public function getTax(): float
+    {
+        $vat = $this->getVat();
+        if (0 == $vat) {
+            return 0;
+        }
+
+        $percent = $vat / 100.00;
+
+        return round($this->getSubtotal() * $percent, 2);
+    }
+
+    /**
+     * @return float
+     */
+    public function getTotal(): float
+    {
+        return $this->getSubtotal() + $this->getTax();
+    }
+
+    /**
+     * @return string
+     */
+    public function getCurrency(): string
+    {
+        return $this->model->getCustomer()->getCurrency();
+    }
+
+    /**
+     * Returns the total amount of worked time in seconds.
+     *
+     * @return int
+     */
+    public function getTimeWorked(): int
+    {
+        $time = 0;
+        foreach ($this->model->getEntries() as $entry) {
+            $time += $entry->getDuration();
+        }
+
+        return $time;
+    }
+}