Add metric for background lumpiness

Author: enourbakhsh

python/lsst/analysis/tools/atools/calexpMetrics.py

@@ -58,6 +58,7 @@ class CalexpSummaryMetrics(AnalysisTool):
         "zeroPoint": "mag",
         "skyBg": "electron",
         "skyNoise": "electron",
+        "skyBgLumpiness": "",  # unitless
         "meanVar": "",  # electron**2
         "astromOffsetMean": "arcsec",
         "astromOffsetStd": "arcsec",