Clearer licensing documentation

[iboB]

The code in this repo is (mostly) under the Apache 2.0 license

HOWEVER:

• The CPU version relies on one of:
  • FFTW3 which is GPLv2 with option to buy a proprietary license from MIT
  • ducc which is GPLv2 with some BSD elements, but if ducc is used, it's not just the BSD elements, but GPLv2 licensed code too. ducc doesn't seem to provide an alternative to GPLv2
• The GPU version uses cufft, which is subject to the standard NVIDIA math lib license.

Not clearly documenting these facts may be a bit misleading to users, who might carelessly assume that finufft is entirely Apache 2.0, whereas, depending on the configuration, it "carries" one of several copyleft licenses with it.

I think this information should be more prominently displayed in the docs and readme.

[ahbarnett]

Good idea. Licensing of dependencies is complicated. As far as I can tell this doesn't need to be in our LICENSE file, but some other notice (eg described in https://ospo.co/blog/practice-tip-licenses-dependencies-and-notice-files/ ). So, we will add such a notice in docs, README.md, etc.

We only use or build the FFT component of DUCC, which seems to be more permissive than "GPLv2 or later", namely, we can choose a 3-clause BSD license. @mreineck can correct us.

Let's get a little input from the other devs. I'm moving this to an Issue.

[iboB]

Some parts of ducc are dual licensed and some are GPL2-only. GPL2-only code from ducc seems to be used. See reply below

[mreineck]

I can confirm that all ducc components that are needed for the FFT part are BSD3/GPLv2+ dual-licensed. Should I have overlooked any source files, I'm happy to fix this.

[iboB]

I haven't gotten into too much detail with the use of ducc (as in which headers are used where, as there are some header-only parts), but the cmake code here:

finufft/cmake/setupDUCC.cmake

Lines 19 to 20 in cbd55cd

	${ducc0_SOURCE_DIR}/src/ducc0/math/gridding_kernel.cc
	${ducc0_SOURCE_DIR}/src/ducc0/math/gl_integrator.cc

explicitly adds two GPL2-only licensed files (highlighted) to the build

[mreineck]

Thanks! That is definitely an oversight, these files should not be needed.

[mreineck]

Fixed on master!