Fixes two-point flux approximation in MPFA-O

[bishtgautam]

• The exponential density function is corrected to ensure that density is constant if the pressure is below the reference pressure. This change required gold files to be updated for tests using exponential density function.
• The face permeability for internal faces in the TPF is modified to be consistent with PFLOTRAN's approach of first converting cell centered tensor permeability to a scalar and then harmonically averaging the two scalar permeabilities. This change required update of gold files for TPF tests.
• The contribution of the gravity term is accounted as a two-point flux (similar to PFLOTRAN's implementation) based on Starnoni et al. (2019)

Fixes #130

[bishtgautam]

It seems that the branch was initially not deployed because it had a conflict. I have now rebased the branch.

[jeff-cohere]

Is the test failure expected (gold file needs updating, perhaps)?

[bishtgautam]

@jeff-cohere The PR is now ready for a second review.

[bishtgautam]

I broke a test and am working on fixing it.

[bishtgautam]

I now broke a bunch of other tests :(

[jeff-cohere]

I now broke a bunch of other tests :(

No sweat! Let me know if you need any extra eyes on anything, and otherwise I'll just re-review when tests are fixed.

[codecov-io]

Codecov Report

Merging #150 (47c3747) into master (b33f25f) will decrease coverage by 0.53%. The diff coverage is 97.53%.

@@            Coverage Diff             @@
##           master     #150      +/-   ##
==========================================
- Coverage   85.25%   84.72%   -0.54%     
==========================================
  Files          48       48              
  Lines       10681    11035     +354     
==========================================
+ Hits         9106     9349     +243     
- Misses       1575     1686     +111     

Impacted Files	Coverage Δ
src/mpfao/3D/tdympfao3D_snes.c	94.50% <57.14%> (-3.15%)	:arrow_down:
demo/transient/transient_snes_mpfaof90.F90	73.33% <75.00%> (-14.91%)	:arrow_down:
src/mpfao/tdympfao.c	91.26% <88.23%> (+1.46%)	:arrow_up:
src/mesh/tdycoremeshutils.c	64.60% <93.75%> (-0.26%)	:arrow_down:
src/mpfao/3D/tdympfao3D_core.c	94.35% <98.14%> (-3.04%)	:arrow_down:
src/mesh/tdycoremesh.c	92.79% <100.00%> (-1.27%)	:arrow_down:
src/mpfao/3D/tdympfao3D_ts.c	99.46% <100.00%> (+0.71%)	:arrow_up:
src/mpfao/3D/tdympfao3D_utils.c	89.97% <100.00%> (+0.05%)	:arrow_up:
src/tdycore.c	82.94% <100.00%> (+0.03%)	:arrow_up:
src/tdyeos.c	90.16% <100.00%> (+3.27%)	:arrow_up:
... and 8 more

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[bishtgautam]

Tests are now fixed.