[1p] Remove implicit folder layer

dumux/porousmediumflow/1p/CMakeLists.txt

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

dumux/porousmediumflow/1p/incompressiblelocalresidual.hh

dumux/porousmediumflow/1p/indices.hh

dumux/porousmediumflow/1p/model.hh

dumux/porousmediumflow/1p/volumevariables.hh

dumux/porousmediumflow/1p/vtkoutputfields.hh

test/mixeddimension/embedded/1p_1p/bloodflowproblem.hh

test/mixeddimension/embedded/1p_1p/tissueproblem.hh

...eddimension/embedded/1p_richards/rootsystemtestproblem.hh

test/mixeddimension/facet/1p_1p/fractureproblem.hh

test/mixeddimension/facet/1p_1p/matrixproblem.hh

test/porousmediumflow/1p/implicit/1pniconductionproblem.hh

test/porousmediumflow/1p/implicit/1pniconvectionproblem.hh

test/porousmediumflow/1p/implicit/1ptestproblem.hh

test/porousmediumflow/1p/implicit/compressible/problem.hh

test/porousmediumflow/1p/implicit/fractureproblem.hh

test/porousmediumflow/1p/implicit/incompressible/problem.hh

...diumflow/1p/implicit/pointsources/1psingularityproblem.hh

test/porousmediumflow/1p/implicit/tubesproblem.hh