improve export to add filtering parameters on result

The csv formatter now add a message about filtering parameters.

The filterInterface has been adapted to render a message.

Export/FilterInterface.php

@@ -33,5 +33,38 @@ namespace Chill\MainBundle\Export;
  */
 interface FilterInterface extends ModifierInterface
 {
+    
+    const STRING_FORMAT = 'string';
+    
+    /**
+     * Describe the filtering action.
+     * 
+     * The description will be inserted in report to remains to the user the 
+     * filters applyied on this report.
+     * 
+     * Should return a statement about the filtering action, like : 
+     * "filtering by date: from xxxx-xx-xx to xxxx-xx-xx" or 
+     * "filtering by nationality: only Germany, France"
+     * 
+     * The format will be determined by the $format. Currently, only 'string' is
+     * supported, later some 'html' will be added. The filter should always 
+     * implement the 'string' format and fallback to it if other format are used.
+     * 
+     * If no i18n is necessery, or if the filter translate the string by himself,
+     * this function should return a string. If the filter does not do any translation,
+     * the return parameter should be an array, where 
+     * 
+     * - the first element is the string, 
+     * - and the second an array of parameters (may be an empty array)
+     * - the 3rd the domain (optional)
+     * - the 4th the locale (optional)
+     * 
+     * Example: `array('my string with %parameter%', ['%parameter%' => 'good news'], 'mydomain', 'mylocale')`
+     * 
+     * @param array $data
+     * @param string $format the format
+     * @return string|array a string with the data or, if translatable, an array where first element is string, second elements is an array of arguments
+     */
+    public function describeAction($data, $format = 'string');
 
 }