These data files are used during unit testing.
The h5 files are generated by
nilmtk.tests.generate_data.create_all(). We do not generate these
files very often (just when the design of the file structure changes).
When nosetests runs, it uses these files. Some files will be modified
by the tests of the cache. These modified files would confuse git so,
when nosetests finishes, it runs teardown_package() in
nilmtk/__init__.py which uses git to reset the nilmtk/data
directory.