added a check for missing kstpkper in DataFile.get_data()
added a check for missing kstpkper in DataFile.get_data()

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