Check if implicit diagnostics file exist before writing the header (BLAST-WarpX#5750)

This is a small fix to prevent the implicit diagnostics file from being
overwritten during a restart.

Author: dpgrote

Source/NonlinearSolvers/NewtonSolver.H

@@ -204,7 +204,9 @@ void NewtonSolver<Vec,Ops>::Define ( const Vec&  a_U,
     this->m_is_defined = true;
 
     // Create diagnostic file and write header
-    if (!this->m_diagnostic_file.empty() && amrex::ParallelDescriptor::IOProcessor()) {
+    if (amrex::ParallelDescriptor::IOProcessor()
+        && !this->m_diagnostic_file.empty()
+        && !amrex::FileExists(this->m_diagnostic_file)) {
 
         std::filesystem::path const diagnostic_path(this->m_diagnostic_file);
         std::filesystem::path const diagnostic_dir = diagnostic_path.parent_path();

Source/NonlinearSolvers/PicardSolver.H

@@ -118,7 +118,9 @@ void PicardSolver<Vec,Ops>::Define ( const Vec&  a_U,
     this->m_is_defined = true;
 
     // Create diagnostic file and write header
-    if (!this->m_diagnostic_file.empty() && amrex::ParallelDescriptor::IOProcessor()) {
+    if (amrex::ParallelDescriptor::IOProcessor()
+        && !this->m_diagnostic_file.empty()
+        && !amrex::FileExists(this->m_diagnostic_file)) {
 
         std::filesystem::path const diagnostic_path(this->m_diagnostic_file);
         std::filesystem::path const diagnostic_dir = diagnostic_path.parent_path();