Fix structure to match Python Packaging User Guide

tests/example_data/bt_Munich_2020061000_72.ict.txt

...mple_data/korusaq-flexpart-dc8_trajectory_20160529_R2.ict

..._data/korusaq-mrg01-HANSEO-KING-AIR_merge_20160507_RA.ict

tests/example_data/korusaq-mrg10-dc8_merge_20160510_R4.ict

tests/example_data/read_and_write.py

tests/example_data/will_fail/AROTAL-RAY_DC8_20040715_R1.ict

tests/example_data/will_fail/LIDARO3_WP3_20040830_R0.ict

tests/test_dataset.py

tests/usage_examples/create_ffi1001.py

tests/usage_examples/create_ffi2110.py

tests/usage_examples/read_ffi1001.py

tests/usage_examples/read_ffi2110.py