Add parabolic turbulence driving mode and mhdturb problem generator

[tianninglyu]

Hi all,

I do some new developments in Athena++ and would like to get feedback from the developers.
The code is here: https://github.com/tianninglyu/athena/tree/turb

Here’s what I’ve added:

Turbulence driving modification

Updated src/fft/turbulence.cpp and src/fft/turbulence.hpp to add a new mode key that allows choosing between powerlaw and parabolic driving.

Implemented a new parabolic driving mode: the driving spectrum is defined over the wavenumber range [nlow, nhigh] with a peak at (nlow + nhigh) / 2.

New mhdturb problem generator

Allows users to specify custom initial conditions in the input file.

Supports enabling a magnetic field (configurable initial B field).

A sample input file is included at inputs/mhd/athinput.mhdturb.

Could you please check if my modification is correct, since this is my first time to modify the source code of athena++?

Thanks for your time and feedback!

Best regards,
Tianning