Adds SetDefault methods to NetworkTables (#54)

There was no way to atomically check for a key in the table,
and then setting if it if non existant. Back before persistent
this was not a problem, however now it is, as its possible for
values to be added before team's robot programs start. This makes
the old method of calling Put*** methods in RobotInit invalid.
This adds SetDefault methods, which do this atomically.

src/ntcore_cpp.cpp

@@ -29,6 +29,10 @@ std::shared_ptr<Value> GetEntryValue(StringRef name) {
   return Storage::GetInstance().GetEntryValue(name);
 }
 
+bool SetDefaultEntryValue(StringRef name, std::shared_ptr<Value> value) {
+  return Storage::GetInstance().SetDefaultEntryValue(name, value);
+}
+
 bool SetEntryValue(StringRef name, std::shared_ptr<Value> value) {
   return Storage::GetInstance().SetEntryValue(name, value);
 }