Added method to get head and ddn unit number from an oc file. If external_unit_dict is passed can also get the name of the files.
Added method to get head and ddn unit number from an oc file. If external_unit_dict is passed can also get the name of the files.

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