Initial HDF5 support

- Initial HDF5 support, fixes #28
  - file handlers are now in the src/io directory
  - add cmake/FindHDF5Lite.cmake to find the H5LT lib
  - link agains HDF5 and HDF5Lite
  - added hdf5 file handler, see io/hdf5_file.hpp
  - added tests for hdf5 file handler, see io/tests
  - removed src/output directory
    - old output/output.hpp is now io/vtk.hpp .
- Initial support for MPI tests:
  - added add_hom3_mpi_test function to root's CMakeLists.txt
  that defines a unit-test file with both name and #of MPI procs.
  - added mpi test to check mpiconfiguration.
- added filesystem support: check and removal of existing files.
- added a simple heap buffer, see  misc/heap_buffer.hpp
- replaced all std::string's with String