include files missing from PyPy installation

[KrisThielemans]

https://github.com/UCL/STIR/pull/1292 creates a GitHub action which installs nipet from pypy in a venv. However, prjb.h is not included as far as I can see. More detail in https://github.com/UCL/STIR/pull/1292#issuecomment-1826453448

[KrisThielemans]

Confirmed on WSL2 with Ubuntu 22.04:

$ python3 -m venv ~/nipet-venv
$ source ~/nipet-venv/bin/activate
$ pip install "nipet>=2"
$ find  /home/kris/nipet-venv -name \*.h|grep nifty
/home/kris/nipet-venv/lib/python3.10/site-packages/niftypet/nimpa/include/nlm.h
/home/kris/nipet-venv/lib/python3.10/site-packages/niftypet/nimpa/include/isub.h
/home/kris/nipet-venv/lib/python3.10/site-packages/niftypet/nimpa/include/cuhelpers.h
/home/kris/nipet-venv/lib/python3.10/site-packages/niftypet/nimpa/include/rsmpl.h
/home/kris/nipet-venv/lib/python3.10/site-packages/niftypet/nimpa/include/conv.h
/home/kris/nipet-venv/lib/python3.10/site-packages/niftypet/nipet/include/scanner_0.h
/home/kris/nipet-venv/lib/python3.10/site-packages/niftypet/nipet/include/def.h
/home/kris/nipet-venv/lib/python3.10/site-packages/niftypet/nipet/include/auxmath.h

[KrisThielemans]

Seems to be missing something like this in prj/CMakeLists.txt

file(GLOB include_files "*.h")
install(FILES  ${include_files} niftypet/${CMAKE_PROJECT_NAME}/include)

Probably required in other directories as well.