Unit Test framework.

[jparkhill]

We need a simple unit test which trains a potential (with all of our model chemistries) on a Formamide, and runs a short MD, just to put the package through it's paces. The training data should be added to the git as a test.

We should then get that running regularly to check if we are breaking the code....

[jparkhill]

I will move all the test*.py files to /examples. and cleaned out some old ones...