Add default copy assignment operator to data classes (#32)

Author: tindy2013

libcron/include/libcron/CronData.h

@@ -21,6 +21,8 @@ namespace libcron
 
             CronData(const CronData&) = default;
 
+            CronData& operator=(const CronData&) = default;
+
             bool is_valid() const
             {
                 return valid;

libcron/include/libcron/CronSchedule.h

@@ -25,6 +25,8 @@ namespace libcron
 
             CronSchedule(const CronSchedule&) = default;
 
+            CronSchedule& operator=(const CronSchedule&) = default;
+
             std::tuple<bool, std::chrono::system_clock::time_point>
             calculate_from(const std::chrono::system_clock::time_point& from) const;