Memory leak observed when using file_observer or time_series_observer

[restenb]

When using the file observer or unbuffered time series observer, memory use of the library is explosive over time. The issue is especially apparent when running a simulation with both, indicating that both these observers have separate memory leak problems.

[kyllingstad]

For time_series_observer, the problem seems to be that when we say "unbuffered" we really mean "unlimited buffer size". ;)

Regarding file_observer, I suspect that the ss_ buffer never gets emptied and keeps growing (unlike the raw data buffers – xxxSamples_ – which get cleared manually). A quick test/fix would be to turn ss_ into a local variable inside persist() instead and see if that improves things.

[kyllingstad]

Fixed by #647.