[freeflow] Remove the atPos functions on the model level

dumux/common/fvproblem.hh

dumux/common/staggeredfvproblem.hh

dumux/freeflow/compositional/staggered/fluxvariables.hh

dumux/freeflow/compositional/staggered/localresidual.hh

dumux/freeflow/navierstokes/staggered/fluxvariables.hh

dumux/freeflow/navierstokes/staggered/localresidual.hh

dumux/freeflow/nonisothermal/localresidual.hh

...eflow/rans/twoeq/lowrekepsilon/staggered/fluxvariables.hh