enh: add 'dcnum yield' pipeline metadata key

dclab/definitions/meta_const.py

@@ -137,6 +137,9 @@
         # segmenter, feature, gate.
         ["dcnum hash", str, "Hash"],
         ["dcnum segmenter", str, "Segmenter ID"],
+        # The event yield the number of events the pipeline produced and
+        # helps identify files that were e.g. exported from original files.
+        ["dcnum yield", str, "Event yield"],
     ],
     # All qpi-related keywords
     "qpi": [