Implement hook_field_access

Currently, access to the field is done in the widget itself, which is not such a good idea imo.

A problem I experienced, I had the field in a vertical tab (fieldgroup), and for users without access to the field, the fieldgroup would appear empty (while it should be hidden.
Checking the html, I saw an empty html container for the radioactivity field.

This can be fixed by adding the following implementation of hook_field_access:

/**
 * Implements hook_field_access().
 */
function radioactivity_field_access($op, $field, $entity_type, $entity, $account) {
  if ($field['type'] == 'radioactivity' && $op == 'edit') {
    return user_access("edit radioactivity");
  }
  return TRUE;
}