Set default values of constants in white_noise_block

[mattpitkin]

Currently, in the white_noise_block function, if vary=False the efac, equad and ecorr values are set as Constant parameters without a given value for that constant. This means that the constant value will default to None. The docstring suggests that the constant values can be set later, but in a function such as model_singlepsr_noise, if white_vary=False these constants don't get set and subsequent likelihood calls on the returned pta object fail due to the constants returning None.

This PR attempts to fix this by setting some default values for the constants. In this case a value of 1.0 for efac and -inf for both equad and ecorr. This (assuming I understand things correctly!) would mean that the likelihood just uses the TOA errors as given as the standard deviations in the likelihood calculation (without any scaling, additional component, or correlation).

See also nanograv/enterprise#379.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 37.29%. Comparing base (d91baf2) to head (70f9434).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #226   +/-   ##
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  Coverage   37.29%   37.29%           
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  Files          20       20           
  Lines        3974     3974           
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  Hits         1482     1482           
  Misses       2492     2492           

Files	Coverage Δ
enterprise_extensions/blocks.py	44.41% <100.00%> (ø)

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