Naming issue in H5 files

Since Scalar/Tensor feature introduction (!5 (merged)) H5 dataset names are not properly set: for instance for a Tensor field U we get :

Poisson_Uout_00000.h5:/U0_t0_X
Poisson_Uout_00000.h5:/U1_t0_X
Poisson_Uout_00000.h5:/U2_t0_X

We should have instead:

Poisson_Uout_00000.h5:/U_t0_X
Poisson_Uout_00000.h5:/U_t0_Y
Poisson_Uout_00000.h5:/U_t0_Z

An improvement as a side effect of solving this issue would be to add a XDMF support for vectors in order to get 'true' vectors in Paraview. This is done by adding XML part for interleace scalar fields:

  <Attribute Name="U0_t0" AttributeType="Vector" Center="Node">
    <DataItem ItemType="Function" Dimensions="32 32 64 3"
              Function=" JOIN( $0,$1,$2 ) ">
 	<DataItem Dimensions="32 32 64" NumberType="Float" Precision="8" Format="HDF" Compression="Raw">
            poisson_Uout_00000.h5:/U_t0_X
	</DataItem>
 	<DataItem Dimensions="32 32 64" NumberType="Float" Precision="8" Format="HDF" Compression="Raw">
	    poisson_Uout_00000.h5:/U_t0_Y
	</DataItem>
 	<DataItem Dimensions="32 32 64" NumberType="Float" Precision="8" Format="HDF" Compression="Raw">
	    poisson_Uout_00000.h5:/U_t0_Z
	</DataItem>
    </DataItem>
  </Attribute>