Add helper functions to create specific readers and writers

Removed the `ReaderEntityFactory::createReader(Type)` method and replaced it by 3 methods:
- `ReaderEntityFactory::createCSVReader()`
- `ReaderEntityFactory::createXLSXReader()`
- `ReaderEntityFactory::createODSReader()`

This has the advantage of enabling autocomplete in the IDE, as the return type is no longer the interface but the concrete type. Since readers may expose different options, this is pretty useful.

Similarly, removed the `WriterEntityFactory::createWriter(Type)` method and replaced it by 3 methods:
- `WriterEntityFactory::createCSVWriter()`
- `WriterEntityFactory::createXLSXWriter()`
- `WriterEntityFactory::createODSWriter()`

Since this is a breaking change, I also updated the Upgrade guide.
Finally, the doc is up to date too.

src/Spout/Writer/Common/Creator/WriterEntityFactory.php

@@ -5,6 +5,8 @@ namespace Box\Spout\Writer\Common\Creator;
 use Box\Spout\Common\Entity\Cell;
 use Box\Spout\Common\Entity\Row;
 use Box\Spout\Common\Entity\Style\Style;
+use Box\Spout\Common\Exception\UnsupportedTypeException;
+use Box\Spout\Common\Type;
 use Box\Spout\Writer\WriterInterface;
 
 /**
@@ -22,14 +24,13 @@ class WriterEntityFactory
      */
     public static function createWriter($writerType)
     {
-        return WriterFactory::create($writerType);
+        return WriterFactory::createFromType($writerType);
     }
 
     /**
      * This creates an instance of the appropriate writer, given the extension of the file to be written
      *
      * @param string $path The path to the spreadsheet file. Supported extensions are .csv, .ods and .xlsx
-     * @throws \Box\Spout\Common\Exception\IOException
      * @throws \Box\Spout\Common\Exception\UnsupportedTypeException
      * @return WriterInterface
      */
@@ -38,6 +39,48 @@ class WriterEntityFactory
         return WriterFactory::createFromFile($path);
     }
 
+    /**
+     * This creates an instance of a CSV writer
+     *
+     * @return \Box\Spout\Writer\CSV\Writer
+     */
+    public static function createCSVWriter()
+    {
+        try {
+            return WriterFactory::createFromType(Type::CSV);
+        } catch (UnsupportedTypeException $e) {
+            // should never happen
+        }
+    }
+
+    /**
+     * This creates an instance of a XLSX writer
+     *
+     * @return \Box\Spout\Writer\XLSX\Writer
+     */
+    public static function createXLSXWriter()
+    {
+        try {
+            return WriterFactory::createFromType(Type::XLSX);
+        } catch (UnsupportedTypeException $e) {
+            // should never happen
+        }
+    }
+
+    /**
+     * This creates an instance of a ODS writer
+     *
+     * @return \Box\Spout\Writer\ODS\Writer
+     */
+    public static function createODSWriter()
+    {
+        try {
+            return WriterFactory::createFromType(Type::ODS);
+        } catch (UnsupportedTypeException $e) {
+            // should never happen
+        }
+    }
+
     /**
      * @param Cell[] $cells
      * @param Style|null $rowStyle