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?