Add support for `spreadinterponly` in finufft #480

Job	Run time
prepare	2s
Build with Pip (ubuntu-22.04, 3.8, llvm, -march=native)	1m 19s
Build with Pip (ubuntu-22.04, 3.8, llvm, -march=x86-64)	1m 18s
Build with Pip (ubuntu-22.04, 3.8, gcc, -march=native)	1m 7s
Build with Pip (ubuntu-22.04, 3.8, gcc, -march=x86-64)	59s
Build with Pip (ubuntu-22.04, 3.11, llvm, -march=native)	1m 12s
Build with Pip (ubuntu-22.04, 3.11, llvm, -march=x86-64)	1m 8s
Build with Pip (ubuntu-22.04, 3.11, gcc, -march=native)	54s
Build with Pip (ubuntu-22.04, 3.11, gcc, -march=x86-64)	1m 13s
Build with Pip (windows-2022, 3.8, msvc, /arch:AVX2)	11m 58s
Build with Pip (windows-2022, 3.8, msvc, /arch:SSE2)	12m 49s
Build with Pip (windows-2022, 3.8, llvm, /arch:AVX2)	12m 48s
Build with Pip (windows-2022, 3.8, llvm, /arch:SSE2)	12m 21s
Build with Pip (windows-2022, 3.11, msvc, /arch:AVX2)	12m 15s
Build with Pip (windows-2022, 3.11, msvc, /arch:SSE2)	12m 36s
Build with Pip (windows-2022, 3.11, llvm, /arch:AVX2)	12m 13s
Build with Pip (windows-2022, 3.11, llvm, /arch:SSE2)	12m 39s
Build with Pip (macos-13, 3.8, llvm, -march=native)	3m 39s
Build with Pip (macos-13, 3.8, llvm, -march=x86-64)	2m 51s
Build with Pip (macos-13, 3.8, gcc-14, -march=native)	1m 55s
Build with Pip (macos-13, 3.8, gcc-14, -march=x86-64)	1m 59s
Build with Pip (macos-13, 3.11, llvm, -march=native)	3m 33s
Build with Pip (macos-13, 3.11, llvm, -march=x86-64)	2m 35s
Build with Pip (macos-13, 3.11, gcc-14, -march=native)	1m 49s
Build with Pip (macos-13, 3.11, gcc-14, -march=x86-64)	2m 43s
	2h 9m 55s

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