reference_psd: fix weirdness in PSDFirKernel

- if a Nyquist override is provided the reported sample rate would be tied to that value instead of the actual frequency bins in the PSD that are used (as it would be without an override).  this patch fixes

gstlal/python/reference_psd.py

@@ -491,9 +491,10 @@ class PSDFirKernel(object):
 		#
 
 		if nyquist is not None:
-			assert nyquist <= sample_rate / 2.
-			sample_rate = nyquist * 2
-			data = data[:int(nyquist / psd.deltaF) + 1]
+			i = int(round((nyquist * 2. - psd.f0) / psd.deltaF))
+			assert i < len(data)
+			data = data[:i + 1]
+			sample_rate = 2 * int(round(psd.f0 + (len(data) - 1) * psd.deltaF))
 
 		#
 		# compute the FIR kernel.  it always has an odd number of