Fcs make getter const (#319)
* make FCS get and conversion-operator methods const
etl::frame_check_sequence has to access methods which can be made const:
value_type value() const
operator value_type() const
* make jenkins_policy::initial and final const
According to the documentation, initial, add and const have to be
tagged as const.
final has to be const now due to the change in the previous commit which
makes the fcs getter methods const.
