potential fix for NFFT module crashing at unload

- similar to a bug in pyFFTW
- no longer use atexit
- cleanup is now registered as a callback to Py_AtExit
- cleanup now also includes fftw_cleanup
update autogen files
version bump
remove useless call to fftw_cleanup
update autogen files
Revert "update autogen files"

This reverts commit 4719b7a7ea4f9a16f2ada618775a917926a89ea3.
Revert "remove useless call to fftw_cleanup"

This reverts commit 8a5d0cb37352b9ac26e6c6c63d9ca6431e8bd049.
fix typo in comment
update autogen files
fix issue #2:

add relative path flag to install instruction for custom libnfft3 library location
fix forgotten find/replace for install flags
update changelog
Merge branch 'develop'