Changes

For hotstaging, sum stages with the same number

If a stage has ullage motors that light at the same time as the main
engine, then hotstaging the _next_ stage does not work properly. Since
both the ullage motors and the "regular" engine have the same KSP stage
number, hotstaging starts when the ullage motors have less than
`HotStagingLeadTime` seconds left to burn, even if the main engine burn
time is much longer.

With this change, all burn times with the same KSP stage are added up
to calculate the `LastNonZeroDVStageBurnTime`.
 (commit: 1bbc559)