HPC regression testing

[Alexey-Voronin]

Here I am taking a first stab at the (non-automated) performance regression test.

List of changes

• Added an HPC regression test suite under regression/hpc/ with a strong-scaling Maxwell cavity example.
• Introduce helper scripts to parse MrHyDE logs (requires verbosity: 10 level output), compute speedup/efficiency, and evaluate parallel scaling.
• Updated readme

Sample scaling output (python3 scripts/runtests.py -k hpc -> scaling_top50.txt)

MrHyDE::AssemblyManager::computeJacRes() - total assembly
--------------------------------------------------------------------------------
MPI procs    Time (s)        Speedup         Efficiency (%) 
--------------------------------------------------------------------------------
1            11.2600         1.0000          100.00         
2            5.6780          1.9831          99.15          
4            2.9840          3.7735          94.34          

MrHyDE::SolverManager::projectDirichlet()
--------------------------------------------------------------------------------
MPI procs    Time (s)        Speedup         Efficiency (%) 
--------------------------------------------------------------------------------
1            9.6810          1.0000          100.00         
2            5.3000          1.8266          91.33          
4            3.4080          2.8407          71.02          

Next steps

I don't expect this to be merged (right away), but perhaps we could start outlining what the performance regression suite should look like for MrHyDE. #10 needs to be merged in first as this builds off of that PR.