Enhance Xdmf3 writer to save block names.

So far block names of multiblocks are not saved. This
result in an important loss of information when loading the file back.

Also remove some warnings.

Change-Id: Ic195ddfc29ad493c2322f4476461dcb0583414a6
Merge topic 'enh_xdmf3_writer' into master

acb40959 Enhance Xdmf3 writer to save block names.