Compute runtime diagnostics

[dwillcox]

Compute runtime diagnostics every so many steps.

To start, can simply implement the sums & averages from reduce_data.py using AMReX reductions.

We'll have to generate code for the reduction kernels & reduction variables, ofc.

Save the reduced data into an HDF5 file with an entry per diagnostic timestep.

Also, when we restart from a checkpoint, if that HDF5 file is in the current directory, we should delete entries at timesteps later than the restart timestep before resuming the simulation.

[dwillcox]

Also:

• Spherical Harmonic decomposition (64 or 128 eq directions is prohibitively slow, even in 1D, with python postprocessing)
• Spatial Fourier transforms (less expensive with python postprocessing)