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.