HDF5Hander now flushes buffer on exit

```
test_flushbuffers (__main__.test_HDF5Handler_ndarrays_resizable) ... Ok
test_flushbuffers_and_trim (__main__.test_HDF5Handler_ndarrays_resizable) ... Ok
test_group_and_dataset_creation (__main__.test_HDF5Handler_ndarrays_resizable) ... Ok
test_group_and_dataset_creation_after_closing (__main__.test_HDF5Handler_ndarrays_resizable) ... Ok
test_group_creation (__main__.test_HDF5Handler_ndarrays_resizable) ... Ok
test_group_creation_after_closing (__main__.test_HDF5Handler_ndarrays_resizable) ... Ok
test_hdf5file_dataset_creation (__main__.test_HDF5Handler_ndarrays_resizable) ... Ok
test_hdf5file_dataset_creation_after_closing (__main__.test_HDF5Handler_ndarrays_resizable) ... Ok
test_multiple_datasets (__main__.test_HDF5Handler_ndarrays_resizable) ... Ok
test_sum_flts_almostequal4 (__main__.test_HDF5Handler_ndarrays_resizable) ... Ok
test_sum_flts_almostequal4_after_closing (__main__.test_HDF5Handler_ndarrays_resizable) ... Ok
test_sum_flts_almostequal5 (__main__.test_HDF5Handler_ndarrays_resizable) ... Ok
test_sum_flts_almostequal5_after_closing (__main__.test_HDF5Handler_ndarrays_resizable) ... Ok
test_sum_flts_almostequal6 (__main__.test_HDF5Handler_ndarrays_resizable) ... Ok
test_sum_flts_almostequal6_after_closing (__main__.test_HDF5Handler_ndarrays_resizable) ... Ok
test_sum_ints (__main__.test_HDF5Handler_ndarrays_resizable) ... Ok
test_sum_ints_after_closing (__main__.test_HDF5Handler_ndarrays_resizable) ... Ok
test_trimming (__main__.test_HDF5Handler_ndarrays_resizable) ... Ok

----------------------------------------------------------------------
Ran 18 tests in 1.079s
```