Move Fornberg coefficients calculations from WarpX to ablastr

[lucafedeli88]

The calculation of Fornberg stencil coefficients is rather general, and it can be shared with other projects of the BLAST family. Therefore, this PR moves the responsible functions into ablastr.

Specifically, the PR does the following:

• 2 new files (FiniteDifference.H and FiniteDifference.cpp) are created under ablastr/math (CMakeLists.txt and Make.package accordingly)
• the static method of the WarpX class getFornbergStencilCoefficients and the ReorderFornbergCoefficients function (originally defined in an anonymous namespace in WarpX.cpp) are moved to these new files, inside the namespace ablastr::math
• the two methods are minimally adapted (e.g., AMREX_ALWAYS_ASSERT_WITH_MESSAGE becomes ABLASTR_ALWAYS_ASSERT_WITH_MESSAGE)
• WarpX.cpp and SpectralKSpace.cpp (where the aforementioned functions were called) are updated

Note that with this PR SpectralKSpace.cpp does not need anymore to include the heavy WarpX.H header.

[EZoni]

Kind of a tangential question, but do we currently have any CI check that compiles with GNU Make instead of CMake? I wonder if code changes like the ones in this file are actually tested automatically.

[EZoni]

I can answer myself. Yes, I see at least one CI workflow here that uses GNU Make:

WarpX/.github/workflows/cuda.yml

Lines 100 to 134 in e0421a1

	# make sure legacy build system continues to build, i.e., that we don't forget
	# to add new .cpp files
	build_nvcc_gnumake:
	name: NVCC 11.8.0 GNUmake
	runs-on: ubuntu-20.04
	if: github.event.pull_request.draft == false
	steps:
	- uses: actions/checkout@v4
	- name: install dependencies
	run: |
	.github/workflows/dependencies/nvcc11-8.sh
	- name: CCache Cache
	uses: actions/cache@v4
	with:
	path: ~/.cache/ccache
	key: ccache-${{ github.workflow }}-${{ github.job }}-git-${{ github.sha }}
	restore-keys: |
	ccache-${{ github.workflow }}-${{ github.job }}-git-
	- name: build WarpX
	run: |
	export CCACHE_COMPRESS=1
	export CCACHE_COMPRESSLEVEL=10
	export CCACHE_MAXSIZE=600M
	ccache -z
	export PATH=/usr/local/nvidia/bin:/usr/local/cuda/bin:${PATH}
	export LD_LIBRARY_PATH=/usr/local/nvidia/lib:/usr/local/nvidia/lib64:/usr/local/cuda/lib64:${LD_LIBRARY_PATH}
	which nvcc || echo "nvcc not in PATH!"
	git clone https://github.com/AMReX-Codes/amrex.git ../amrex
	cd ../amrex && git checkout --detach 78bdf0faabc4101d5333ebb421e553efcc7ec04e && cd -
	make COMP=gcc QED=FALSE USE_MPI=TRUE USE_GPU=TRUE USE_OMP=FALSE USE_FFT=TRUE USE_CCACHE=TRUE -j 4
	ccache -s
	du -hs ~/.cache/ccache

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