Install pytraj for development

cd /path/to/pytraj/root/folder

# compile Cython pyx files
python setup.py build_ext -i

# install pytraj in developer mode
# Only need to do once
python -m pip install -e .

# Why? Basically if you want to edit `.py` file in pytraj, 
# you do not need to do `python setup.py install` again.

# Note: If you edit Cython's `.pyx` file, 
# you still need to `python setup.py build_ext -i` again.