External HDF5 dirs in STATISMO_INCLUDE_DIRS

In statismo-config.cmake, explicitly set HDF5_INCLUDE_DIR and
HDF5_INCLUDE_DIRS instead of  @CMAKE_INSTALL_PREFIX@/include/hdf5 and
@CMAKE_INSTALL_PREFIX@/include/hdf5/cpp. This allows
STATISMO_INCLUDE_DIRS to contain the path of the HDF5 include dirs also
in case of external HDF5 libraries. In the case of HDF5 libraries
installed by Statismo, HDF5_INCLUDE_DIR and HDF5_INCLUDE_DIRS are set to
be the in the install directory of statismo, rather than in the build
directory.
Avoid conflict with VTK for H5cpp.h

Explicitly prepend Statismo include directories to VTK include
directories in order to avoid conflict in HDF5 library ("unresolved
external")