[iterators][segment_iterator] correct comments
[iterators][segment_iterator] correct comments; fix code length to be
at most 80 characters;
[iterators][segment_iterator] move range_segment_iterator class to
detail::segment_iterator namespace; move bg::segment_iterator_value_type
to bg::detail::segment_iterator::value_type;
[iterators][segment_iterator] fix file-inclusion guarding macro to
reflect true file path
[iterators][segment_iterator] move implementation of bg::segment_iterator_type
to bg::detail::segment_iterator namespace; rename the class segment_iterator_type
to iterator_type;
[iterators][segment_iterator] update implementation of segment_iterator
due to the change of namespace and name of the segment_iterator_type class;
make constructors of bg::segment_iterator class inline; change the return type
of segments_begin and segments_end to be segment_iterator<Geometry const>;
[test][iterators][segment_iterator] change the return type of segments_begin
and segments_end to be segment_iterator<Geometry const>