Add flow executable with simpler MaterialLawManager

[multitalentloes]

This PR adds a flow executable that will use the simple EclMaterialLawManager in opm-common to enable GPU support of property evaluation on simple flow cases

[GitPaean]

I know it is still under draft, while a small suggestion, maybe a more descriptive name can be better.

[GitPaean]

Thanks for changing the title, but I meant the name of the executable flow_simple. Hopefully the name can give people some hints on what this executable is doing.