declare front porch variable in common params, set default value in exp params, remove from simparams

Author: lynnt20

sbndcode/WireCell/cfg/pgrapher/common/params.jsonnet

@@ -156,6 +156,11 @@ local wc = import "wirecell.jsonnet";
     // Parameters related to simulation, not given elsewhere.
     sim : {
 
+        // The "absolute" time (ie, relative to trigger time?) that the lower edge
+        // of final readout tick #0 should correspond to.  This is a
+        // "fixed" notion.
+        tick0_time: 0,
+
         // The number of impact bins per wire region gives the
         // granularity of the simulation convolution in the transverse
         // dimension.  Typically should match what the granularity at

sbndcode/WireCell/cfg/pgrapher/experiment/sbnd/params.jsonnet

@@ -120,6 +120,11 @@ base {
         // For running in LArSoft, the simulation must be in fixed time mode. 
         fixed: true,
 
+        // The "absolute" time (ie, relative to trigger time?) that the lower edge
+        // of final readout tick #0 should correspond to.
+        // this is the default value unless overridden with extVar in main
+        tick0_time: -200 * wc.us,
+
         // Open the ductor's gate a bit early.
         local response_time_offset = $.det.response_plane / $.lar.drift_speed,
         local response_nticks = wc.roundToInt(response_time_offset / $.daq.tick),

sbndcode/WireCell/cfg/pgrapher/experiment/sbnd/simparams.jsonnet

@@ -76,13 +76,6 @@ base {
     continuous: false,
     fluctuate: false,
 
-    // The "absolute" time (ie, relative to trigger time?) that the lower edge
-    // of final readout tick #0 should correspond to.  This is a
-    // "fixed" notion.
-    // note: should be overriden by extVar in main jsonnet to maintain consistency
-    // with the larsoft clock service
-    local tick0_time = -200*wc.us, 
-
     //ductor : super.ductor {
     //    start_time: $.daq.start_time - $.elec.fields.drift_dt + $.trigger.time,
     //},