addded usergroup.php to /form/field/ and /form/header/ folders

Need to use Joomla's #__usergroups in my FOF XML forms.

This PR pertains to the plural XML view only, as the singular XML view works wonderfully when using:
```
<field name="usergroup"
               type="usergroup"
               label="COM_FOOBAR_VIEW_FIELD_LABEL"
               required="true"
        />
```

Although FOF does not have its own field specification for #__usergroups, Joomla does have its own. What a wonderful example of "Framework on Framework"!

The plural view needs FOF love, though. 

So, based on FOF's accesslevel form, I created this usergroup field. 

Unlike accesslevel, there is no concept of "allowed" usergroup. So, line 128 in /form/field/usergroup.php, and its if..then statements, should be deleted. As well, /form/header/usergroup.php should be deleted as well. 

Instead of deleting the code, I commented it out. 

I am imposing on you (Nicholas et al) to do this deletion because I confess to confusion with #__viewlevels  and #__usergroups.  

Thank you for taking the time to review this PR,
-Bob