Add support for `spreadinterponly` in finufft #476

Job	Run time
prepare	3s
Build with Pip (ubuntu-22.04, 3.8, llvm, -march=native)	1m 24s
Build with Pip (ubuntu-22.04, 3.8, llvm, -march=x86-64)	1m 14s
Build with Pip (ubuntu-22.04, 3.8, gcc, -march=native)	58s
Build with Pip (ubuntu-22.04, 3.8, gcc, -march=x86-64)	58s
Build with Pip (ubuntu-22.04, 3.11, llvm, -march=native)	1m 26s
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 4s
Build with Pip (ubuntu-22.04, 3.11, gcc, -march=x86-64)	1m 0s
Build with Pip (windows-2022, 3.8, msvc, /arch:AVX2)	11m 45s
Build with Pip (windows-2022, 3.8, msvc, /arch:SSE2)	11m 43s
Build with Pip (windows-2022, 3.8, llvm, /arch:AVX2)	12m 30s
Build with Pip (windows-2022, 3.8, llvm, /arch:SSE2)	11m 59s
Build with Pip (windows-2022, 3.11, msvc, /arch:AVX2)	11m 51s
Build with Pip (windows-2022, 3.11, msvc, /arch:SSE2)	11m 49s
Build with Pip (windows-2022, 3.11, llvm, /arch:AVX2)	11m 58s
Build with Pip (windows-2022, 3.11, llvm, /arch:SSE2)	14m 7s
Build with Pip (macos-13, 3.8, llvm, -march=native)	3m 9s
Build with Pip (macos-13, 3.8, llvm, -march=x86-64)	3m 34s
Build with Pip (macos-13, 3.8, gcc-14, -march=native)	2m 10s
Build with Pip (macos-13, 3.8, gcc-14, -march=x86-64)	1m 50s
Build with Pip (macos-13, 3.11, llvm, -march=native)	2m 14s
Build with Pip (macos-13, 3.11, llvm, -march=x86-64)	2m 28s
Build with Pip (macos-13, 3.11, gcc-14, -march=native)	2m 16s
Build with Pip (macos-13, 3.11, gcc-14, -march=x86-64)	2m 15s
	2h 6m 51s

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