refactor: use the Buffer and BoundingBox types to perform the communication packing / unpacking in Axis & ConstrainedTransport classes

[svalat]

What

Replace the hand made buffer packing by using BoundingBox and Buffer into the Axis class in preparation to a larger refactoring to latter use the Exchanger class into Axis to manage the communications.

Process made in 3 steps

1. Add the new communication way (duplicate)
2. Compare both while running
3. Removing old.

Needed a add UnpackJDirSymetric() in Buffer class.

Validation

• 32 MPI process on MHD/AxisFluxTube in bound check + dual communication (new & old compared in flight by asserts)
• Test suite on MHD/AxisFluxTube MPI.

Squash

I let the steps commits in the branch, but they can be squashed for the merge in develop.