Improve tests

[nebfield]

The existing tests do some unpleasant things like os.chdir to a module scoped fixture directory:

fraposa_pgsc/tests/test_fraposa.py

Lines 77 to 82 in 6c62aa5

	def test_fraposa(ref_data, filt_id, args):
	with patch('sys.argv', args):
	cwd = os.getcwd()
	os.chdir(ref_data)
	main()
	os.chdir(cwd)

Also, the tests don't cover some functions like split input data well. Perhaps integrating codecov would help us find the dusty places in the code.