[1pnc] Remove implicit folder level

dumux/porousmediumflow/1pnc/CMakeLists.txt

dumux/porousmediumflow/1pnc/implicit/CMakeLists.txt

dumux/porousmediumflow/1pnc/indices.hh

dumux/porousmediumflow/1pnc/model.hh

dumux/porousmediumflow/1pnc/volumevariables.hh

dumux/porousmediumflow/1pnc/vtkoutputfields.hh

dumux/porousmediumflow/1pncmin/implicit/indices.hh

dumux/porousmediumflow/1pncmin/implicit/model.hh

dumux/porousmediumflow/1pncmin/implicit/properties.hh

dumux/porousmediumflow/1pncmin/implicit/volumevariables.hh

...porousmediumflow/1pnc/implicit/1p2cniconductionproblem.hh

...porousmediumflow/1pnc/implicit/1p2cniconvectionproblem.hh

test/porousmediumflow/1pnc/implicit/1p2ctestproblem.hh