Introduce MultiSplineBase and MultiSplineOffload classes #7096

Job	Run time
macos (macOS-GCC14-NoMPI-Real)	14m 32s
spack-centos-stream (GCC11-NoMPI-Debug-Real)	18m 38s
linux (GCC9-NoMPI-Debug-Real)	16m 50s
spack-centos-stream (GCC11-NoMPI-NoOMP-Real)	17m 25s
spack-centos-stream (GCC11-NoMPI-NoOMP-Complex)	18m 7s
linux (GCC9-NoMPI-NoOMP-Real)	15m 12s
spack-centos-stream (GCC11-NoMPI-Sandbox-Real)	4m 6s
linux (GCC9-NoMPI-NoOMP-Complex)	15m 26s
linux (GCC9-MPI-Sandbox-Real)	3m 40s
linux (GCC9-NoMPI-Sandbox-Complex)	3m 40s
linux (GCC9-MPI-Gcov-Real)	25m 12s
linux (GCC9-MPI-Gcov-Complex)	25m 51s
linux (GCC12-NoMPI-Werror-Real)	18m 13s
linux (GCC12-NoMPI-Werror-Complex)	18m 43s
linux (GCC12-NoMPI-Werror-Real-Mixed)	17m 50s
linux (GCC12-NoMPI-Werror-Complex-Mixed)	18m 17s
linux (Clang14-NoMPI-ASan-Real)	24m 12s
linux (Clang14-NoMPI-ASan-Complex)	24m 4s
linux (Clang14-NoMPI-UBSan-Real)	24m 34s
linux (Clang16-NoMPI-Offload-Real)	22m 19s
linux (Clang16-NoMPI-Offload-Complex)	23m 2s
	6h 9m 53s

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