Add FiniteElementFunction as new Attribute ItemType
This PR is a part of GSoC project for visualization of FEniCS (fenicsproject.org) finite element functions through XDMF format.
New attribute to Attribute named "ItemType" is added. In such case "ElementFamily", "ElementDegree", "ElementCell" specify finite element function in a FEniCS way (UFL specification). In addition, since data are not centered in a standard way new centering "Other" is added.
Multiple DataItems must be stored under single Attribute - this is dealt with mostly on LOKI level.