Add functionality to selectively suppress key warnings

For unknown keys.

{
  "dont_warn_unused_keys": [
    "key1",
    "key3"
  ],
  "key1": 5,
  "key2": {
    "dont_warn_unused_keys": [
      "key1"
    ],
    "key1": 5,
    "key2": 6
  },
  "key3": {
    "dont_warn_unused_keys": [
      "key1"
    ],
    "key1": 5,
    "key2": 6
  }
}

Will yield the warning:

[Series] The following parts of the global JSON config remains unused:
{
  "key2": {
    "dont_warn_unused_keys": [
      "key1"
    ],
    "key1": 5,
    "key2": 6
  },
  "key3": {
    "key2": 6
  }
}

TODO: Better errors when parsing, documentation

include/openPMD/auxiliary/JSON_internal.hpp

@@ -162,6 +162,9 @@ namespace json
             nlohmann::json *positionInShadow,
             SupportedLanguages originallySpecifiedAs,
             bool trace);
+
+        void init();
+        static void init(nlohmann::json &original, nlohmann::json &shadow);
     };
 
     template <typename Key>

src/auxiliary/JSON.cpp

@@ -53,7 +53,9 @@ TracingJSON::TracingJSON(
     , m_shadow(std::make_shared<nlohmann::json>())
     , m_positionInOriginal(&*m_originalJSON)
     , m_positionInShadow(&*m_shadow)
-{}
+{
+    init();
+}
 
 TracingJSON::TracingJSON(ParsedConfig parsedConfig)
     : TracingJSON{
@@ -158,7 +160,31 @@ TracingJSON::TracingJSON(
     , m_positionInOriginal(positionInOriginal)
     , m_positionInShadow(positionInShadow)
     , m_trace(trace)
-{}
+{
+    init();
+}
+
+void TracingJSON::init()
+{
+    if (m_originalJSON)
+    {
+        init(*m_originalJSON, *m_shadow);
+    }
+}
+
+void TracingJSON::init(nlohmann::json &original, nlohmann::json &shadow)
+{
+    if (original.is_object() && original.contains("dont_warn_unused_keys"))
+    {
+        auto suppress_warnings_for_these = original.at("dont_warn_unused_keys")
+                                               .get<std::vector<std::string>>();
+        for (auto const &key : suppress_warnings_for_these)
+        {
+            init(original[key], shadow[key]);
+        }
+        shadow["dont_warn_unused_keys"] = nlohmann::json();
+    }
+}
 
 namespace
 {