SW-561: Convert the sample window first to a double before a division

I am afraid that the 0Hz central frequency can be attributed to an integer
divided by a double.  This should at least remove the uncertainty.

RTCP/Cobalt/OutputProc/src/TBB_Dipole.cc

@@ -397,7 +397,7 @@ namespace LOFAR
                 currentSubBand.centralFrequency;
             currentSubBand.dataSet->centralFrequencyUnit().value = "Hz";
             currentSubBand.dataSet->timeResolution().value =
-                SPECTRAL_TRANSFORM_SIZE /
+                static_cast< double >(SPECTRAL_TRANSFORM_SIZE) /
                     (frame.header.sampleFreq * 1000000.0);
             currentSubBand.dataSet->timeResolutionUnit().value =
                 "s";