Make itkStatismoConfig.h first included header.

Fix needed to avoid version conflicts due to duplicate headers, when statismo_ITK is used together with an external hdf5 installation.
Additional include guard to prevent HDF5Cpp.h to be included when itkH5Cpp is used.
Avoid the use of ITK's HDF5 libraries for vtk representerTests.
Merge pull request #69 from statismo/ITKHdf5

Minor fixes concerning the use of ITK's own hdf5 libraries.