Validate a GTFS

    :param validator: a ValidationReport
    :param (bool) skip_relations: skip validation of relations between entities (e.g. stop_times to stops)
    :return:

def validate(self, validator=None, skip_relations=False):
    """Validate a GTFS

    :param validator: a ValidationReport
    :param (bool) skip_relations: skip validation of relations between entities (e.g. stop_times to stops)
    :return:
    """
    validator = validation.make_validator(validator)
    self.log('Loading...')
    self.preload()
    # required
    required = [
      'agency',
      'stops',
      'routes',
      'trips',
      'stop_times',
      'calendar'
    ]
    for f in required:
      self.log("Validating required file: %s"%f)
      data = self.read(f)
      for i in data:
        i.validate(validator=validator)
        if skip_relations is False:
          i.validate_feed(validator=validator)
    # optional
    optional = [
      'calendar_dates',
      'fare_attributes',
      'fare_rules',
      'shapes',
      'frequencies',
      'transfers',
      'feed_info'
    ]
    for f in optional:
      self.log("Validating optional file: %s"%f)
      try:
        data = self.read(f)
      except KeyError, e:
        data = []
      for i in data:
        i.validate(validator=validator)
        if skip_relations is False:
          i.validate_feed(validator=validator)
    return validator