Making the check on sigma for FisherSky class more relax (allowing 0 …

…value) (#4994)

* Making the check on sigma for FisherSky class more relax (allowing 0
value)

* Changing the error message for sigma check

---------

Co-authored-by: jacquot <jacquot@>

pycbc/distributions/sky_location.py

@@ -98,9 +98,9 @@ def __init__(self, **params):
                 'The mean declination must be between '
                 f'-π/2 and π/2, {mean_dec} rad given'
             )
-        if sigma <= 0 or sigma > 2 * numpy.pi:
+        if sigma < 0 or sigma > 2 * numpy.pi:
             raise ValueError(
-                'Sigma must be positive and smaller than 2π '
+                'Sigma must be non-negative and smaller than 2π '
                 '(preferably much smaller)'
             )
         if sigma > 0.35: