ParmParse::addfile needs Init

`ParmParse` can be used before AMReX is initialized for most
functionality, which is greatly helpful for us. One function that
cannot be used before init is reading from a file, because it needs
an MPI context to broadcast.

Add a clean error message instead of a segfault.

Src/Base/AMReX_ParmParse.cpp

@@ -538,6 +538,11 @@ read_file (const char* fname, std::list<ParmParse::PP_entry>& tab)
     //
     if ( fname != nullptr && fname[0] != 0 )
     {
+#ifdef AMREX_USE_MPI
+        if (!amrex::Initialized())
+            amrex::Error("read_file: AMReX must be initialized");
+#endif
+
         Vector<char> fileCharPtr;
         std::string filename = fname;
         ParallelDescriptor::ReadAndBcastFile(filename, fileCharPtr);
@@ -1068,6 +1073,11 @@ ParmParse::prefixedName (const std::string& str) const
 
 void
 ParmParse::addfile (std::string const& filename) {
+#ifdef AMREX_USE_MPI
+    if (!amrex::Initialized())
+        amrex::Error("ParmParse::addfile: AMReX must be initialized");
+#endif
+
     auto l = std::list<std::string>{filename};
     auto file = FileKeyword;
     addDefn(file,