Fix MacOS compilation

Author: dsarmany

src/multio/action/statistics/operations/OperationWithData.h

@@ -64,7 +64,7 @@ class OperationWithData : public Operation {
 
     void load(std::shared_ptr<StatisticsIO>& IOmanager, const StatisticsOptions& opt) override {
         if (needRestart_) {
-            std::uint64_t sz;
+            std::size_t sz;
             std::string fname = restartFileName();
             IOmanager->readSize(fname, sz);
             values_.resize(sz);

src/multio/action/statistics/operations/OperationWithDeaccumulatedData.h

@@ -94,7 +94,7 @@ class OperationWithDeaccumulatedData : public Operation {
 
     void load(std::shared_ptr<StatisticsIO>& IOmanager, const StatisticsOptions& opt) override {
         if (needRestart_) {
-            std::uint64_t sz;
+            std::size_t sz;
             std::string fname = restartFileName();
             IOmanager->readSize(fname, sz);
             values_.resize(sz);