mostly docstrings
	modified:   cell_tracker/objects.py
	new file:   cell_tracker/tests/test_analysis.py
	new file:   cell_tracker/tests/test_detection.py
	new file:   cell_tracker/tests/test_graphics.py
	new file:   cell_tracker/tests/test_objects.py
	deleted:    cell_tracker/tests/test_track_cells.py
	new file:   cell_tracker/utils.py
	new file:   notebooks/Tests.ipynb
	modified:   notebooks/nb_init.py