(v1.0) Re-organized ExcelBuilderAction.php.twig

Added  a ```getValueForCell``` protected function so that extending the values format for the cells will be easier.

For instance if we need to format the value of a field which we know it's a currency, we can easily extends this method and add those special cases. E.g.:
```php
protected function getValueForCell($field, $rowData)
{
    switch($field) {
        case 'price':
        case 'productionCost':
            $getter = 'get'.ucfirst($field);
            $data = number_format($rowData->$getter(), 2, ',', '.');
            break;
        default:
            $data = parent::getValueForCell($field, $rowData);
            break;
    }
    return $data;
}
```

I don't know if this is the best example, but this can also be used to render boolean field with some text other than ```TRUE/FALSE```.
Yes, we should keep the `getValueForCell` as the "defeault" formatter. But it's probably best if we keep it `protected` (someone may want to overwrite it).

About related fields.. if you have
```php
<?php
class User
{
  /**
   * @var string
   */
  $name;
}

class Post
{
  /**
   * @var User 
   */
  protected $user;
}
```

then in `Post-generator.yml` we will have a column named `user`

So, to add custom formatting, simply add `formatUserValue()`

We can also define a virtual column `user.name`. In this case to add custom formatting we add `formatUserNameValue()`.

Though right now, I can't find where `classify` is defined. Not in Admingenerator, nor in TwigExtensions, also no word of it in base Twig doc... hmm...