Instability for Boundary Fuel Model

[mcgratta]

This case test.txt has similar properties to CSIRO Case F19. It runs only because I increased the thermal conductivity of everything to a high value, but even then it is very unstable. The char forms and is consumed very quickly leading to wild temperature swings.

[ericvmueller]

With the changes I have here to update the definition of RHO_S and scale conductivity by PACKING_RATIO, the example you have works when shrinking is allowed.

Case_F19_BFM_crude from CSIRO also works, but Case_C064_BFM_crude fails. However, if I use the little bluestem grass pyrolysis model rather than the generic one then both appear to work, albeit with somewhat high surface temperatures...

Another option might be to continue to enforce some MINIMUM_BURNOUT_TIME for now to prevent things becoming unstable.

[ericvmueller]

The test case works with the above commits ...but we may want to leave this issue open while I explore removing MINIMUM_BURNOUT_TIME from validation cases