deprecate NO_FIELDS

[oxinabox]

This deprecates NO_FIELDS to NoTangent().
Note that this is not the same as the 0.9 behavour which had it as ZeroTangent().
but it is close, and is the behavour we want going forward
(which is why we decided that this had to be in breaking change. But we ment to include it in the 0.10.0 release).

[codecov-commenter]

Codecov Report

Merging #361 (9d99e1f) into master (2c69796) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #361   +/-   ##
=======================================
  Coverage   89.26%   89.26%           
=======================================
  Files          12       12           
  Lines         475      475           
=======================================
  Hits          424      424           
  Misses         51       51           

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