Add a machine name process plugin

This patch adds the test but i'm getting PHP Fatal error (calling method on non-object) from inside MachineName::getTransliteration. My test string is not over 128 characters long as indicated in the summary, but it's unclear to me why this would be necessary. Either there really is a bug or i'm missing something here. Setting to needs review, hoping for feedback so I can finish this up.

Here are some more points and suggestions.

1. +++ b/core/modules/migrate/lib/Drupal/migrate/Plugin/migrate/process/MachineName.php
   @@ -0,0 +1,49 @@
   +   * @var \Drupal\Core\Transliteration\PHPTransliteration
   ...
   +   * @return \Drupal\Core\Transliteration\PHPTransliteration
   
   +++ b/core/modules/migrate/tests/Drupal/migrate/Tests/process/MachineNameTest.php
   @@ -0,0 +1,89 @@
   +class MachineNameTest extends MigrateProcessTestCase {
   

   desc line missing

2. +++ b/core/modules/migrate/tests/Drupal/migrate/Tests/process/MachineNameTest.php
   @@ -0,0 +1,89 @@
   + * Contains
   

   Incomplete

3. +++ b/core/modules/migrate/tests/Drupal/migrate/Tests/process/MachineNameTest.php
   @@ -0,0 +1,89 @@
   +  protected $mapJoinable = FALSE;
   

   @var block missing.

4. +++ b/core/modules/migrate/tests/Drupal/migrate/Tests/process/MachineNameTest.php
   @@ -0,0 +1,89 @@
   +   * Test machine name transformation of non-alphanumeric characters.
   

   Tests

5. +++ b/core/modules/migrate/tests/Drupal/migrate/Tests/process/MachineNameTest.php
   @@ -0,0 +1,89 @@
   +    $human_name_ascii = 'foo2, the.bar;2*&the%baz!YEE____HaW ';
   

   YEE___HaW :D

6. +++ b/core/modules/migrate/tests/Drupal/migrate/Tests/process/MachineNameTest.php
   @@ -0,0 +1,89 @@
   +namespace Drupal\migrate\Plugin\migrate\process;
   +
   +use Drupal\Component\Transliteration\TransliterationInterface;
   +
   +class TestMachineName extends MachineName {
   +  public function setTransliteration(TransliterationInterface $transliteration) {
   +    $this->transliteration = $transliteration;
   +  }
   +}
   

   All kind of doxgen missing i.e. desc block for code,class and function.

+++ b/core/modules/migrate/lib/Drupal/migrate/Plugin/migrate/process/MachineName.php
@@ -0,0 +1,49 @@
+ * This plugin sets a destination property based on multiple sources and a map.
...
+class MachineName extends PluginBase implements MigrateProcessInterface {

I don't really think the description matches to the actual code.

Thank you!

+++ b/core/modules/migrate/tests/Drupal/migrate/Tests/process/MachineNameTest.php
@@ -0,0 +1,93 @@
+  /**
+   * @var bool
+   */
+  protected $mapJoinable = FALSE;
+

This variable is a) not documented (neither here nor in the base class — it's @TODO), and b) not used in the code below. I mentioned this to chx, and he said that there is already a follow-up issue to further clean-up mapJoinable at #2152131: Clean up mapJoinable, so this will just get roped into that. Normally this variable is needed, when an actual migration is being tested, but it is not here, so it is superfluous for now, but may not always be, so it's best to keep it here to be safe.

With that follow-up filed, we are probably good to go here for now, so...

Committed and pushed to 8.x. Thanks!