python callback at restart (#5549)

This PR adds a python callback option after initialization at restart,
enabling users to read user-defined checkpoint vars that could be
written using the python call afterdiagnostics that already exists

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Author: RevathiJambunathan

Python/pywarpx/callbacks.py

@@ -280,6 +280,7 @@ def callfuncsinlist(self, *args, **kw):
     "loadExternalFields": {},
     "beforeInitEsolve": {},
     "afterInitEsolve": {},
+    "afterInitatRestart": {},
     "afterinit": {},
     "beforecollisions": {},
     "aftercollisions": {},
@@ -406,6 +407,16 @@ def installafterInitEsolve(f):
     installcallback("afterInitEsolve", f)
 
 
+# ----------------------------------------------------------------------------
+def callfromafterInitatRestart(f):
+    installcallback("afterInitatRestart", f)
+    return f
+
+
+def installafterInitatRestart(f):
+    installcallback("afterInitatRestart", f)
+
+
 # ----------------------------------------------------------------------------
 def callfromafterinit(f):
     installcallback("afterinit", f)

Source/Initialization/WarpXInitData.cpp

@@ -610,6 +610,9 @@ WarpX::InitData ()
             AddExternalFields(lev);
         }
     }
+    else {
+        ExecutePythonCallback("afterInitatRestart");
+    }
 
     if (restart_chkfile.empty() || write_diagnostics_on_restart) {
         // Write full diagnostics before the first iteration.