Clean up around BsplineAllocator.hpp and MultiSpline.hpp #7098

Job	Run time
linux (GCC9-NoMPI-Debug-Real)	15m 53s
macos (macOS-GCC14-NoMPI-Real)	14m 27s
linux (GCC9-NoMPI-NoOMP-Real)	15m 22s
spack-centos-stream (GCC11-NoMPI-Debug-Real)	19m 20s
linux (GCC9-NoMPI-NoOMP-Complex)	15m 36s
spack-centos-stream (GCC11-NoMPI-NoOMP-Real)	18m 52s
linux (GCC9-MPI-Sandbox-Real)	3m 35s
spack-centos-stream (GCC11-NoMPI-NoOMP-Complex)	18m 27s
linux (GCC9-NoMPI-Sandbox-Complex)	3m 43s
spack-centos-stream (GCC11-NoMPI-Sandbox-Real)	4m 20s
linux (GCC9-MPI-Gcov-Real)	25m 11s
linux (GCC9-MPI-Gcov-Complex)	25m 49s
linux (GCC12-NoMPI-Werror-Real)	19m 3s
linux (GCC12-NoMPI-Werror-Complex)	19m 25s
linux (GCC12-NoMPI-Werror-Real-Mixed)	17m 50s
linux (GCC12-NoMPI-Werror-Complex-Mixed)	18m 10s
linux (Clang14-NoMPI-ASan-Real)	25m 13s
linux (Clang14-NoMPI-ASan-Complex)	24m 30s
linux (Clang14-NoMPI-UBSan-Real)	24m 4s
linux (Clang16-NoMPI-Offload-Real)	22m 10s
linux (Clang16-NoMPI-Offload-Complex)	22m 46s
	6h 13m 46s

Provide feedback