Fix structure to match Python Packaging User Guide

CHANGES.md

INSTALL.md

INSTALL.rst

MANIFEST.in

README.md

docs/source/conf.py

docs/source/index.rst

docs/source/usage.rst

pyproject.toml

setup.cfg

setup.py

src/icartt/__init__.py

src/icartt/dataset.py

tests/example_data/AR_DC8_20050203_R0.ict

tests/example_data/BB-FLUX_CU-SOF_20180808_R2.ict

tests/example_data/DC8-20160517.ict

tests/example_data/Dongdaemun_NIER_20160520_RA.ict

tests/example_data/HOX_DC8_20040712_R0.ict

tests/example_data/NOx_RHBrown_20040830_R0.ict

tests/example_data/PAVE-AR_DC8_20050203_R0.ict