Add support for `spreadinterponly` in finufft #479

Job	Run time
prepare	3s
Build with Pip (ubuntu-22.04, 3.8, llvm, -march=native)	1m 17s
Build with Pip (ubuntu-22.04, 3.8, llvm, -march=x86-64)	1m 6s
Build with Pip (ubuntu-22.04, 3.8, gcc, -march=native)	1m 1s
Build with Pip (ubuntu-22.04, 3.8, gcc, -march=x86-64)	57s
Build with Pip (ubuntu-22.04, 3.11, llvm, -march=native)	1m 11s
Build with Pip (ubuntu-22.04, 3.11, llvm, -march=x86-64)	1m 6s
Build with Pip (ubuntu-22.04, 3.11, gcc, -march=native)	1m 1s
Build with Pip (ubuntu-22.04, 3.11, gcc, -march=x86-64)	1m 2s
Build with Pip (windows-2022, 3.8, msvc, /arch:AVX2)	11m 32s
Build with Pip (windows-2022, 3.8, msvc, /arch:SSE2)	12m 18s
Build with Pip (windows-2022, 3.8, llvm, /arch:AVX2)	12m 37s
Build with Pip (windows-2022, 3.8, llvm, /arch:SSE2)	12m 57s
Build with Pip (windows-2022, 3.11, msvc, /arch:AVX2)	11m 26s
Build with Pip (windows-2022, 3.11, msvc, /arch:SSE2)	11m 9s
Build with Pip (windows-2022, 3.11, llvm, /arch:AVX2)	12m 32s
Build with Pip (windows-2022, 3.11, llvm, /arch:SSE2)	12m 25s
Build with Pip (macos-13, 3.8, llvm, -march=native)	3m 42s
Build with Pip (macos-13, 3.8, llvm, -march=x86-64)	3m 48s
Build with Pip (macos-13, 3.8, gcc-14, -march=native)	2m 35s
Build with Pip (macos-13, 3.8, gcc-14, -march=x86-64)	1m 57s
Build with Pip (macos-13, 3.11, llvm, -march=native)	3m 19s
Build with Pip (macos-13, 3.11, llvm, -march=x86-64)	3m 0s
Build with Pip (macos-13, 3.11, gcc-14, -march=native)	1m 54s
Build with Pip (macos-13, 3.11, gcc-14, -march=x86-64)	1m 37s
	2h 7m 32s

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