Use DST-2 DST-3 in FFTW Poisson solver

[AlexanderSinn]

This PR adds the DST-2 / DST-3 Poisson solver from #1305 to the FFTW backend and direct Poisson solver. FFTW already provides the needed DSTs.

Like on GPU, the Poisson solver is chose automatically based on if the grid has an even or odd resolution.

I tested the new Poisson solver with MR on CPU and the boundary is indeed set correctly.

• Small enough (< few 100s of lines), otherwise it should probably be split into smaller PRs
• Tested (describe the tests in the PR description)
• Runs on GPU (basic: the code compiles and run well with the new module)
• Contains an automated test (checksum and/or comparison with theory)
• Documented: all elements (classes and their members, functions, namespaces, etc.) are documented
• Constified (All that can be const is const)
• Code is clean (no unwanted comments, )
• Style and code conventions are respected at the bottom of https://github.com/Hi-PACE/hipace
• Proper label and GitHub project, if applicable

[MaxThevenet]

Thanks for this PR!