Fixes ESSS#71

This is a first attempt, including unit tests.
Also some minor issues in DataFrameRegressionFixture were fixed,
which was used as a starting point for this PR.
The new fixture only uses NumPy, so also the error message makes
no use for pandas for formatting the errors.

Changes:
- Add statistics of differing elements.
- Add support for complex and string arrays.
- Add support for 0-D arrays.
- Add more unit tests.
- Also raise AssertionError when new items are present in the dictionary.
- Simplified code and clarified error messages.

- Add missing files
- Remove unused ones
- Facilitate regeneration of all NPZ files.