Condense two nonnegative functions

[purva-thakre]

Description

Based on the discussion in #723, this PR combines is_nonegative and is_doubly_nonnegative into 1 function.

Changes

Notable changes that this PR has either accomplished or will accomplish. Feel free to add more lines to the itemized list
below.

• Change 1

Checklist

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.7%. Comparing base (de9a087) to head (24ace04).
Report is 316 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff            @@
##           master    #731     +/-   ##
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- Coverage    97.7%   97.7%   -0.1%     
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  Files         166     165      -1     
  Lines        3250    3245      -5     
  Branches      795     795             
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- Hits         3177    3172      -5     
  Misses         48      48             
  Partials       25      25             

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[vprusso]

One minor comment, but LGTM! Nice call on doing this given that the same was done for the stochastic case!