[Opm] Reorganization of modules: opm-polymer and opm-autodiff

[Atgeirr Rasmussen]

Dear OPM community,

I would like to simplify the module structure of OPM by moving the content of opm-polymer
into opm-autodiff. I intend to initially use the exact same paths as before, so initially after
the move the opm-autodiff module will contain a new opm/polymer directory.

This will benefit testing and maintenance, as we have quite often experienced that upstream
changes in opm-autodiff have not been accounted for in opm-polymer (including by myself).

My intention after the move is to change the module name (perhaps to opm-flow or opm-simulator),
and reorganize the directory structure of the module to make it more logical, but I'd like to make the
move first.

If no-one objects I intend to do the reorganization early next week.

Atgeirr Flø Rasmussen