Refactoring flow `StackVariables` to use `DerivOffset`

[jafranc]

As it is used in isothermalCompositionalMultiphaseFVMKernel or MultifluidBase, derivative offset helps readability and would made extension easier.

The refactoring though left out for now

• dTrans_dDisJump as this derivative is multidimensional
• Hybrid Kernels with pressure both at cell center and face.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 73.91304% with 6 lines in your changes are missing coverage. Please review.

Project coverage is 53.25%. Comparing base (65064b0) to head (2147533).

Files	Patch %	Lines
...ls/SinglePhasePoromechanicsConformingFractures.hpp	0.00%	2 Missing :warning:
...nels/SinglePhasePoromechanicsEmbeddedFractures.hpp	0.00%	2 Missing :warning:
...malSinglePhasePoromechanicsConformingFractures.hpp	0.00%	1 Missing :warning:
...ermalSinglePhasePoromechanicsEmbeddedFractures.hpp	0.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #3085 +/- ## ======================================== Coverage 53.25% 53.25% ======================================== Files 989 989 Lines 83542 83543 +1 ======================================== + Hits 44493 44494 +1 Misses 39049 39049 ```

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