CWMakeFakeData: transient signals can be time-shifted by up to 1/2 sampling step

Service Desk request from Reinhard.Prix@aei.mpg.de :

Looking at XLALCWMakeFakeData(), in the loop over injection signals (simplified):

Tseries_sum = XLALCreateREAL4TimeSeries ( detPrefix, &firstGPS,
fHeterodyne, dt, &lalStrainUnit, numSteps );
for ( UINT4 iInj = 0; iInj < numPulsars; iInj ++ ) {
    ...
    Tseries_i = XLALGenerateCWSignalTS ( pulsarParams, site,
signalStartGPS, signalDuration, fSamp, fMin ...);
    ...
    Tseries_sum = XLALAddREAL4TimeSeries ( Tseries_sum, Tseries_i );
}

there is a problem, because XLALAddREAL4TimeSeries() adds two timeseries with non-aligned time steps by simply rounding each sample to the nearest bin of the series to be added to!

In lal/src/tools/TimeSeries_source.c:150:

REAL8 Delta_epoch = XLALGPSDiff(&arg2->epoch, &arg1->epoch);
i = floor(Delta_epoch / arg1->deltaT + 0.5);

For typical sampling steps of narrow-band timeseries dt = 1/fSamp ~ O(seconds), this would be many signal cycles, and therefore an arbitrary phase shift, which generally can be different for different detectors (if they have different start-times), and lead to non-coherent signals.

Fortunately this is not a problem for continous CWs, which all start at the common epoch 'signalStartGPS = firstGPS', but would affect transient signals.

The fix would be simply to make sure the Tseries_i start-times are aligned to the timeseries bins of Tseries_sum.

Furthermore I would actually suggest to add a failure check to XLALAddREAL4TimeSeries(), as this kind of leniency does not seem to be justified ever for such a critical function.

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