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

@@           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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