removed dependency on disc from VTKWriter.

There are two versions of each of the methods addCellData/addVertexData
1) From a vector container of appropriate size
2) From a VTKFunction object that allows local evaluation
   => The P0Function and P1Function define adapters to VTKFunctions
   In Fact, any GridFunction can be wrapped up as an argument

[[Imported from SVN: r4392]]