Add a helper class to check if a type is complex

[IvanaGyro]

Many refactoring works including #504, #508, and #509 use this change.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 16.78%. Comparing base (72453ca) to head (fca444d).
Report is 40 commits behind head on dev-master.

Additional details and impacted files

@@              Coverage Diff               @@
##           dev-master     #510      +/-   ##
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+ Coverage       16.60%   16.78%   +0.17%     
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  Files             221      221              
  Lines           48490    53173    +4683     
  Branches        20272    19822     -450     
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+ Hits             8053     8923     +870     
- Misses          36143    39940    +3797     
- Partials         4294     4310      +16     

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[manuschneider]

Why not call the function to use is_complex directly? So I would suggest
is_complex_v -> is_complex
is_complex -> is_complex_type
or something similar. This would affect #508 and #509 as well.

Generally, a short comment for functions and classes about what they do or how they are used is always welcome. Like

// is_complex checks if a data type is of type std::complex
// usage: is_complex<T> returns true or false for a data type T

[ianmccul]

The terminology is_complex<T> for the structure and is_complex_v<T> for the value matches the C++ convention of the type_traits, eg is_integral -> is_integral_v, is_pointer -> is_pointer_v etc. It is perhaps slightly unfortunate naming, but the structure metafunctions were added (and got the nice name) in C+11, before template variables were added in C++14, so we got the variables with the _v suffix. For the same reason, metafunctions that return a type conventionally have a helper template alias that adds _t suffix.

The terminology @IvanaGyro has used also matches a proposal to add is_complex and is_complex_v to the C++ standard library, and the boost version.