Added make target for `black`.

help:

	@echo " "

	@echo " Usage:"

	@echo "   make black          # re-format python files following 'black' tool"

	@echo "   make docu           # build documentation"

	@echo "   make clean          # remove temporary files"

	@echo "   make clean-all      # remove temporary files and generated documentation"

	@echo " "

# re-format python files,

# use local maximum line-length convention ..

black:

	black --line-length=99 py

# generate documentation:

docu:

	@echo " "