Add offline multiFab converter and tool to generate npy file

[bq9958]

Summary

Updates the offline tools in Tools/multiFabConverter/offline/ for use with MPI-enabled pyAMReX: a parallel visualization script (main.py), a batch plt→npy conversion script (script_plt_to_npz.py), and a single README documenting both.

Changes

main.py

• Targets MPI-enabled pyAMReX: boxes are distributed across ranks via AMReX's DistributionMapping when reading the plot file.
• Prints per-rank box count (Rank r: n/N boxes) after loading the plt.
• Gathers the full domain with input_mf[:] (collective) then, on rank 0, extracts a 2D slice of x_velocity at a given z and saves slice_z.png.

script_plt_to_npz.py

• Single-process script (no mpirun) to convert one or more plt files to a stacked .npy array (n_samples, ncomp, nx, ny).
• Supports multiple plot file paths; if a path is a directory containing plt* subdirs, uses the latest one.
• Options: -o output path, -l level, -c component names (default: x_velocity, y_velocity, avg_pressure).

README.md

• Describes both scripts in English: purpose, dependencies (including MPI pyAMReX install from conda-forge), usage, arguments, and output.

Testing

• main.py: run with MPI and a valid plt path, e.g. mpirun -np 4 python main.py <path_to_plt>; check per-rank box output and slice_z.png on rank 0.
• script_plt_to_npz.py: e.g. python script_plt_to_npz.py <path_to_plt> -o out.npy; verify out.npy shape and components.