[Trajectory] Implement operator= to fix compilation warning.

Author: florent-lamiraux

include/sot/core/trajectory.hh

@@ -98,7 +98,12 @@ class SOT_CORE_EXPORT timestamp {
     if ((secs_ != other.secs_) || (nsecs_ != other.nsecs_)) return false;
     return true;
   }
-  friend std::ostream &operator<<(std::ostream &stream, const timestamp &ats) {
+ timestamp &operator=(const timestamp &other) {
+   secs_ = other.secs_;
+   nsecs_ = other.nsecs_;
+   return *this;
+ }
+ friend std::ostream &operator<<(std::ostream &stream, const timestamp &ats) {
     stream << ats.secs_ + 0.000001 * (long double)ats.nsecs_;
     return stream;
   }