add py_access_lock_ to CorrectionSet

Author: lgray

include/correction.h

@@ -6,6 +6,7 @@
 #include <variant>
 #include <map>
 #include <memory>
+#include <mutex>
 #include "correctionlib_version.h"
 
 namespace correction {
@@ -305,6 +306,7 @@ class CorrectionSet {
     std::map<std::string, Correction::Ref> corrections_;
     std::map<std::string, CompoundCorrection::Ref> compoundcorrections_;
     std::string description_;
+    std::mutex py_access_lock_;
 };
 
 } // namespace correction