added functions to get the DataTypeId for a given data type
write PixelDimension and ImageDimension in HDF5 file
new representer vtkStandardImageRepresenter. Changed examples accordingly
Moved helper classes vtkPixel and vtkPoint into vtkHelper.h. A
Added vtkHelper class with utilities used in several vtkRepresenters.
Changed data format (load/save) such that pixelValues is now in subgroup pointData.
changed dataTypeID definitions to have a reasonable ordering
Changed includes from vtkPoints and vtkPixels to vtkHelper
Made the fiels DatasetType and Version of the representer an attribute, instead of a string.

In order to be able to write the attributes, the Load/Save method would have to be changed
to take a H5::Group instead of a H5::CommonFG (which is also more correct)