[v1.4.5] Additions, improvements and fixes to tests

New unit tests for the function used to load nodes were provided by Mr. @BMaxV via pull request. I @KennedyRichard also refactored and complemented them.

I also used the opportunity to make some small improvements in the doctests and to fix one of them.

In order to make it more convenient to store test data, .zip files found in Nodezator's source are now also included in the source distribution. That is, they are downloaded in conjunction with the Python modules when someone installs Nodezator.