Create directory for tests for each module

	new file:   ETusefuls/test_ETusefuls.py
	new file:   ETwaves/test_ETwaves.py
	deleted:    test_ETusefuls.py
	deleted:    test_ETwaves.py
Separate tests of different functions from the same .py file
Manually deleted some files
Delete null_stream.py from modules/
Rename ETusefuls.py ETangles_rotations.py
Time function calls
Time ETwaves.principal_polarisation_axes_0Psi