make: add help and fix test launch
make: add help and fix test launch

* add help as a default target
* document main targets
* tweak test launches to work: pytest, nosetests, unittests

SKA-ScienceDataProcessor/algorithm-reference-library,SKA-ScienceDataProcessor/algorithm-reference-library,SKA-ScienceDataProcessor/algorithm-reference-library,SKA-ScienceDataProcessor/algorithm-reference-library,SKA-ScienceDataProcessor/algorithm-reference-library