Adds test to check SFT_timestamps exists

If SFTs where not loaded, i.e. if injectSources was used, no timestamps
are generated. In such a case don't attempt to used them.

pyfstat/core.py

@@ -547,6 +547,7 @@ class SemiCoherentSearch(BaseSearchClass, ComputeFstat):
             record_segments=False):
         """ Returns twoF or ln(BSGL) semi-coherently at a single point """
 
+        if hasattr(self, 'SFT_timestamps'):
             if self.tboundaries[0] < self.SFT_timestamps[0]:
                 logging.debug(
                     'Semi-coherent start time {} before first SFT timestamp {}'