- discuss functions like 
    is_infinite
    is_empty 
    start_is_close 
    end_is_open

    cardinality ?

- Tests for DateTime::Span

- Tests for DateTime::SpanSet

- New constructors-from-duration API:

    $set2 = DateTime::Span->new( 
        start => $date1, 
        end => $date2 );                 # OK
    $set2 = DateTime::Span->new( 
        start => $date1, 
        duration => $dt_duration );      # TODO

    $set1 = DateTime::SpanSet->new( 
        sets => [ $dt_set, $dt_set ] );  # OK
    $set1 = DateTime::SpanSet->new( 
        spans => [ $dt_span, $dt_span ], 
        duration => $dt_duration );      # TODO
    $set1 = DateTime::SpanSet->new(
        dates => [ $dt_span, $dt_span ],
        duration => $dt_duration );      # TODO


