Revert info_per_bit tolerance to 1.5

The xbitinfo implementation uses a tolerance factor of 1.5. I lowered the tolerance to 1.1, because I was getting poor results with my test data, but it was pointed out that the problem was that my test data were quantized. Quantization is an open issue with xbitinfo too, and we should address it first there, then patch the codec.

numcodecs/bitinfo.py

@@ -274,7 +274,7 @@ def get_keepbits(info_per_bit, inflevel=0.99):
 
 def _cdf_from_info_per_bit(info_per_bit):
     """Convert info_per_bit to cumulative distribution function"""
-    tol = info_per_bit[-4:].max() * 1.1  # reduced from 1.5
+    tol = info_per_bit[-4:].max() * 1.5
     info_per_bit[info_per_bit < tol] = 0
     cdf = info_per_bit.cumsum()
     return cdf / cdf[-1]