JK BMS: updateFrom: skip data points with equal value

this changes the interpretation of the timestamp in data containers that
are merely updated from other data containers: this is the oldest
timestamp known where the value was as recorded by the data point in its
respective container.

the data container constructed from an answer will -- naturally -- have
the timetamps of its data points set to the time they were constructed.

include/JkBmsDataPoints.h

@@ -185,6 +185,10 @@ class DataPoint {
         std::string const& getUnitText() const { return _strUnit; }
         uint32_t getTimestamp() const { return _timestamp; }
 
+        bool operator==(DataPoint const& other) const {
+            return _value == other._value;
+        }
+
     private:
         std::string _strLabel;
         std::string _strValue;

src/JkBmsDataPoints.cpp

@@ -48,7 +48,14 @@ std::string dataPointValueToStr(tCells const& v) {
 void DataPointContainer::updateFrom(DataPointContainer const& source)
 {
     for (auto iter = source.cbegin(); iter != source.cend(); ++iter) {
-        _dataPoints.erase(iter->first);
+        auto pos = _dataPoints.find(iter->first);
+
+        if (pos != _dataPoints.end()) {
+            // do not update existing data points with the same value
+            if (pos->second == iter->second) { continue; }
+
+            _dataPoints.erase(pos);
+        }
         _dataPoints.insert(*iter);
     }
 }