rate.BinnedLnPDF.at_centres(): silence warnings

- it's normal for PDFs to be 0, but that leads to complains about divide-by-zero and invalid value in subtract in the .normalize() method of BinnedLnPDF.
- this patch silences

lal/python/lal/rate.py

@@ -2203,6 +2203,7 @@ class BinnedLnPDF(BinnedDensity):
 		return InterpBinnedArray(self)
 
 	def at_centres(self):
+		with numpy.errstate(divide = "ignore", invalid = "ignore"):
 			return numpy.log(super(BinnedLnPDF, self).at_centres()) - self.norm
 
 	def marginalize(self, dim):