Don't force 0 left power balance on 0 power #81

Technically, a zero total power results in a divide-by-zero, so there
is not valid correct result. We were defaulting, in this case, to zero
left-balance, but this doesn't really make any more sense than any
other value.

Now, we default to 50% left power if both left and right are equal (ie
left os 50% of 0, and right is 50% of 0), which makes a lot more sense
visually (eg when rendered in an application).

We now also handle special case where left and right might be equally
opposite in sign (eg -10 and +10), which still results in a zero total
power. So here, we use a left-balance of 0 or 100%, if the left is the
lesser (negative) or greater (positive) of the two respectively.

src/polar/v2/trainingsession.cpp

@@ -2185,8 +2185,9 @@ QStringList TrainingSession::toHRM(const bool rrDataOnly) const
                         // In case we only have right power, not left.
                         const int powerLeft = havePowerLeft ? currentPowerLeft : currentPower - currentPowerRight;
                         // Convert the left and right powers into a left-right balance percentage.
-                        const int leftBalance = (currentPower == 0) ? 0 :
-                            qRound(100.0 * (float)powerLeft / (float)currentPower);
+                        const int leftBalance = (currentPower == 0)
+                            ? (currentPowerLeft == currentPowerRight) ? 50 : (currentPowerLeft < currentPowerRight) ? 0 : 100
+                            : qRound(100.0 * (float)powerLeft / (float)currentPower);
                         if (leftBalance > 100) {
                             qWarning() << "leftBalance of " << leftBalance << "% is greater than 100%";
                         }