Unity Build Support

[ax3l]

As discussed offline, we want to add opt-in support for Unity builds in AMReX. Unity builds combine all .cpp files into a single one (a single translation unit, TU). This is useful for performance optimization, computer science experiments, or differentiable programming.

Examples:

• Add ImpactX_UNITY_BUILD BLAST-ImpactX/impactx#861
• Add WarpX_UNITY_BUILD BLAST-WarpX/warpx#5702