Fix output spec to expect a directory name

RU/cris-l1b-ru

 #!/bin/bash -e
 
 usagexit() {
-	echo >&2 "usage: cris-l1b-ru {SNPP,J1} L1B-FILENAME RU-FILENAME"
-	echo >&2 "docker usage: docker run --rm -v\$PWD:\$PWD -w\$PWD gitlab.ssec.wisc.edu:5555/cris_l1b/user cris-l1b-ru {SNPP,J1} L1B-FILENAME RU-FILENAME"
+	echo >&2 "usage: cris-l1b-ru {SNPP,J1} L1B-FILENAME RU-OUTPUT-DIR"
+	echo >&2 "docker usage: docker run --rm -v\$PWD:\$PWD -w\$PWD gitlab.ssec.wisc.edu:5555/cris_l1b/user cris-l1b-ru {SNPP,J1} L1B-FILENAME RU-OUTPUT-DIR"
 	exit 1
 }
 
@@ -10,7 +10,6 @@ abspath() {
     python -c 'import sys,os; print("\n".join([os.path.abspath(x) for x in sys.argv[1:]]))' "$@"
 }
 
-
 test -d "$RU_HOME" || echo >&2 "${Oops:?RU_HOME is not set}"
 test -d "$MCRROOT" || echo >&2 "${Oops:?MCRROOT must point to Matlab runtime}"
 SAT="$1"
@@ -20,7 +19,7 @@ OUTPUT_FILE="$3"
 
 test -n "$SAT" || usagexit
 test -f "$INPUT_FILE" || echo >&2 "${Oops:?${INPUT_FILE} is not a file}"
-test -n "$OUTPUT_FILE" || echo >&2 "${Oops:?please specify an output filename}"
+test -d "$OUTPUT_FILE" || echo >&2 "${Oops:?please specify an output directory}"
 
 export MATLABPATH="$RU_HOME"
 # always limit the blast radius of LD_LIBRARY_PATH to as small a space as possible