refactor: clarify new gravity, clean up kernel flags usage, move CFL computations to FVM solver

[paveltomin]

• clarify new gravity as requested here fix: make new gravity treatment from #3337 an option #3467 (comment)
• clean up kernel flags usage in flow solver to avoid bugs: the basic idea is to set the flags outside of kernels create and launch to avoid mismatch between different version (isothermal/thermal etc) when one flag is set here but is missing there
• move CFL computations to FVM solver where it belongs

[rrsettgast]

@paveltomin Yes. this is what i meant. Could you:

1. add more detail in the description and explain the options a bit
2. think about adding a more detailed description in the documentation

[paveltomin]

@paveltomin Yes. this is what i meant. Could you:

1. add more detail in the description and explain the options a bit
2. think about adding a more detailed description in the documentation

done, please have a look

[paveltomin]

new rebaselines needed:

********************************************************************************
Error: /Problem/Solvers/compflow
	Group has a child 'useNewGravity' in the baseline file but not the file to compare.
********************************************************************************
********************************************************************************
Error: /Problem/Solvers/compflow
	Group has a child 'gravityDensityScheme' in the file to compare but not the baseline file.
********************************************************************************

[paveltomin]

@rrsettgast is it good enough?

[CusiniM]

LGTM

[codecov]

Codecov Report

Attention: Patch coverage is 29.89130% with 129 lines in your changes missing coverage. Please review.

Project coverage is 56.83%. Comparing base (b3592de) to head (4cf2b6e).
Report is 1 commits behind head on develop.

Files with missing lines	Patch %	Lines
...csSolvers/fluidFlow/CompositionalMultiphaseFVM.cpp	23.13%	103 Missing ⚠️
...sSolvers/fluidFlow/CompositionalMultiphaseBase.hpp	0.00%	5 Missing ⚠️
...s/fluidFlow/kernels/compositional/IHUPhaseFlux.hpp	0.00%	4 Missing ⚠️
...vers/fluidFlow/kernels/compositional/CFLKernel.cpp	0.00%	3 Missing ⚠️
...ysics/CompositionalMultiphaseReservoirAndWells.cpp	0.00%	3 Missing ⚠️
...ositional/DiffusionDispersionFluxComputeKernel.hpp	0.00%	2 Missing ⚠️
...idFlow/kernels/compositional/FluxComputeKernel.hpp	50.00%	2 Missing ⚠️
...al/ThermalDiffusionDispersionFluxComputeKernel.hpp	0.00%	2 Missing ⚠️
...rs/multiphysics/CoupledReservoirAndWellKernels.hpp	0.00%	2 Missing ⚠️
...sSolvers/fluidFlow/CompositionalMultiphaseBase.cpp	83.33%	1 Missing ⚠️
... and 2 more

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #3486      +/-   ##
===========================================
+ Coverage    56.82%   56.83%   +0.01%     
===========================================
  Files         1154     1154              
  Lines        99983    99954      -29     
===========================================
- Hits         56818    56812       -6     
+ Misses       43165    43142      -23     

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