Potential thread interference using CssiSpaceWeatherData

When using the UnscentedKalmanEstimator, an exception (out of range date for ephemeris) was thrown by the CssiSpaceWeatherData class. Analysis under debugger is tricky in the multithreaded context of the UKF, but it seems to indicate a potential problem of thread interference on the previousParam and nextParam fields. The exception thrown should never occur in standard use (line 170 in the bracketDate method of the CssiSpaceWeatherData class).

I suspect that a similar problem might occur with the MarshallSolarActivityFutureEstimation class, but there is no exception to catch this case.

Not being an expert on concurrency in Java, I don't know a solution to this problem.