XLALSimAddInjectionREAL8TimeSeries() XLALSimAddInjectionREAL4TimeSeries()

preformance improvement:  don't pad input time series if not doing
frequency-domain transformation
XLALREAL8TimeSeriesInterp:  new kernel, perf. improvments

- switch from Hann-windowed sinc kernel to top hat-windowed sinc kernel.
  testing shows the frequency response is more flat
- cache the kernel and only recompute if the fractional part of the sample
  offset changes by more than the no-op threshold
- increase the no-op threshold to 0.01 samples from 0.001 samples