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.

tests/Spout/Writer/ODS/WriterPerfTest.php

@@ -2,7 +2,6 @@
 
 namespace Box\Spout\Writer\ODS;
 
-use Box\Spout\Common\Type;
 use Box\Spout\TestUsingResource;
 use Box\Spout\Writer\Common\Creator\WriterEntityFactory;
 use PHPUnit\Framework\TestCase;
@@ -38,8 +37,7 @@ class WriterPerfTest extends TestCase
         $this->createGeneratedFolderIfNeeded($fileName);
         $resourcePath = $this->getGeneratedResourcePath($fileName);
 
-        /** @var Writer $writer */
-        $writer = WriterEntityFactory::createWriter(Type::ODS);
+        $writer = WriterEntityFactory::createODSWriter();
         $writer->setShouldCreateNewSheetsAutomatically(true);
 
         $writer->openToFile($resourcePath);