Counterflow Diffusion Flame Validation

[justint1997]

Added section to Validation.rst including validation results comparing PeleLMeX counterflow flame centerline profiles to that of Cantera.

[baperry2]

Thanks for adding this @justint1997! The documentation additions look great.

Could you also add the input file (and any modifications the the pelelmex_prob* files if applicable) that you used to run this case to the PeleLMeX/Exec/Production/CounterFlow directory? And if the python script to run the Cantera flame and generate the comparison plots is simple enough, it would be great if you could add that too.

[baperry2]

Can you add a README with a short description of the case and how to run it with the cold flow first and then ignition? See an example here: https://github.com/AMReX-Combustion/PeleLMeX/blob/development/Exec/RegTests/FlameSheet/README.md

Then I think this is good.