Resolve "Header generation is triggered every time "setup.py" is called"

Closes #12 (closed)

Headers are now generated only, if the install_headers option is given.

(.test) $ python ./setup.py --name
lisaconstants

(.test) $ python ./setup.py --version
1.0.1

(.test) $ python ./setup.py install
...
...
...
Installed /path/to/.test/lib/python3.9/site-packages/lisaconstants-1.0.1-py3.9.egg
Processing dependencies for lisaconstants==1.0.1
Finished processing dependencies for lisaconstants==1.0.1

(.test) $ python ./setup.py install_headers
creating /path/to/.test/include/python3.9
creating /path/to/.test/include/python3.9/lisaconstants
installing C++ header file -> /path/to/.test/include/python3.9/lisaconstants/lisaconstants.hpp
installing C header file -> /path/to/.test/include/python3.9/lisaconstants/lisaconstants.h