Fix Xdmf2 writer: array type was not respected. Also save block name.

An bug lead to fetch the wrong data array attribute and thus to
save scalar arrays as vectors and vice-versa.
For multiblock data input, the block name is also saved in order to
avoid loosing this important information.

Change-Id: I7860230cce2fa67ee2c9c0627ecb2871c6124936
Merge topic 'fix_xdmf2_writer' into master

2cc28ea5 Fix Xdmf2 writer: array type was not respected. Also save block name.