[io] adapt all porousmediumflow and geomechanics models
Rename `VtkOutputFields` to `IOFields`. Add `primaryVariableName` functions, don't use centralized names yet. Add required template parameters. Adapt properties correspondingly.
Rename `VtkOutputFields` to `IOFields`. Add `primaryVariableName` functions, don't use centralized names yet. Add required template parameters. Adapt properties correspondingly.