Style, documentation and unit tests

This pull request:

* cleans up the style of the code to meet the [PEP8 standard](http://www.python.org/dev/peps/pep-0008/).
* minor adjustments to the doc strings of the new functions.
* unit tests for the cylp processing functions.
* adds a check for the h5py module so that Py-ART can be imported without h5py, this fixed the CI tests.

With these changes I think the original pull request can be merged.  Thanks for the contributions!