bug fix: check that f_ssm is not  closed at the start of each kper writing loop
bug fix: check that f_ssm is not  closed at the start of each kper writing loop

brclark-usgs/flopy,brclark-usgs/flopy,mrustl/flopy,jentjr/flopy,aleaf/flopy,aleaf/flopy,jentjr/flopy,bdestombe/flopy-1