These .py files contain unittests (using the standard Python library
unittest). Use 'make tests' to run all of the tests. To run under a 
different version of python, say python3 for example, use 
'make PYTHON=python3 tests'.

Some tests involve random numbers and so may occasionally --- less than 1
in 100 runs --- fail due to rare multi-sigma fluctuations; rerun the tests
if they do fail.

 