Specialize angle and sign

[jishnub]

These were being specialized in ApproxFunBase, but it's best to avoid the type-piracy and move the methods to this package. These are useful for complex intervals.

julia> iv = 0..im
0 + 0im..0 + 1im

julia> angle(iv) ≈ pi/2
true

[codecov]

Codecov Report

Base: 99.16% // Head: 99.17% // Increases project coverage by +0.01% 🎉

Coverage data is based on head (b9b2b5b) compared to base (c88d162).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #133      +/-   ##
==========================================
+ Coverage   99.16%   99.17%   +0.01%     
==========================================
  Files           3        3              
  Lines         240      243       +3     
==========================================
+ Hits          238      241       +3     
  Misses          2        2              

Impacted Files	Coverage Δ
src/IntervalSets.jl	98.36% <100.00%> (+0.04%)	⬆️

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[hyrodium]

What's the mathematical meaning of 0..im? (x-ref: #47)

[hyrodium]

I believe we don't need support for complex numbers. Please reopen this issue or comment here if you have any additional thoughts.