new rule: copysign

[RainerHeintzmann]

As opposed to the previous discussion the correct rule for copysign(y,x) should probably be:

@scalar_rule copysign(y, x) (ifelse(signbit(x)!=signbit(y), -one(y), +one(y)), NoTangent())

. You can also find the arguments there why NoTangent() is preferred over 0.
The testing gets into problems, if too close to zero for x or y, due to the non-continuity of this function.

[codecov-commenter]

Codecov Report

Merging #502 (8ee5ccc) into master (262be3c) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #502   +/-   ##
=======================================
  Coverage   98.13%   98.13%           
=======================================
  Files          22       22           
  Lines        2356     2357    +1     
=======================================
+ Hits         2312     2313    +1     
  Misses         44       44           

Impacted Files	Coverage Δ
src/rulesets/Base/fastmath_able.jl	98.33% <100.00%> (+0.01%)	⬆️

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[oxinabox]

thanks!