Move DECam precursor step to its own pipeline

pipelines/DECam/DRP-Merian.yaml

@@ -1,6 +1,12 @@
 description: |
   The DRP pipeline specialized for the DECam instrument, developed against the
   Merian dataset.
+
+  Prior to running subsets or tasks in this pipeline, the DECam prerequisite
+  task isrForCrosstalkSources must be run. More information on that task can be
+  found in the isrForCrosstalkSources.yaml pipeline file.
 imports:
-  - $DRP_PIPE_DIR/pipelines/_ingredients/DECam/DRP.yaml
-  - $ANALYSIS_DRP_DIR/pipelines/analysis_drp_plots.yaml
+  - location: $DRP_PIPE_DIR/pipelines/_ingredients/DECam/DRP.yaml#isrForCrosstalkSources
+    exclude:
+      - isrForCrosstalkSources
+  - location: $ANALYSIS_DRP_DIR/pipelines/analysis_drp_plots.yaml

pipelines/DECam/isrForCrosstalkSources.yaml

@@ -0,0 +1,27 @@
+description: |
+  The DECam prerequisite task isrForCrosstalkSources.
+
+  The isrForCrosstalkSources task generates crosstalk sources for
+  ISR/inter-chip crosstalk by applying overscan correction on raw frames. A new
+  dataset is written, which should be used as an input for further DECam data
+  processing.
+
+  This task is intended to be run once, prior to initial data processing.
+instrument: lsst.obs.decam.DarkEnergyCamera
+tasks:
+  isrForCrosstalkSources:
+    class: lsst.ip.isr.IsrTask
+    config:
+      connections.outputExposure: overscanRaw
+      doOverscan: true
+      doAssembleCcd: false
+      doBias: false
+      doCrosstalk: false
+      doVariance: false
+      doLinearize: false
+      doDefect: false
+      doNanMasking: false
+      doDark: false
+      doFlat: false
+      doFringe: false
+      doInterpolate: false