Add support for EDGEFLAG attributes in legacy reader & writer.

So far edgeflag attributes were not serialized/unserialized.
This miss prevent edgeflags to be transfered in ParaView
data delivery mechanism - and avoid edges to be correctly
represented by the client in client/server mode.

Change-Id: I64d6c06f8fa6d1c92f9156d973b28c863b9efa1f
Merge topic 'add_edgeflags_io_support' into master

8efe5ef2 Add support for EDGEFLAG attributes in legacy reader & writer.