Add Vector(2|3|4)<T>::NaN to easily create invalid vectors

[chapulina]

🎉 New feature

Summary

I often need to create invalid vectors to indicate that something isn't initialized or became invalid. There are ways of expressing this with null pointers or std::nullopt, but using NaNs makes it convenient to do without involving the creation and destruction of new vectors.

This also plays well with the existing IsFinite and Correct functions. Note that none of this applies to integer types.

Test it

Take a look at the added tests.

Checklist

• Signed all commits for DCO
• Added tests
• Added example and/or tutorial
• Updated documentation (as needed)
• Updated migration guide (as needed)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
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[codecov]

Codecov Report

Merging #222 (564de1c) into ign-math6 (82acdff) will increase coverage by 0.17%.
The diff coverage is n/a.

@@              Coverage Diff              @@
##           ign-math6     #222      +/-   ##
=============================================
+ Coverage      99.22%   99.39%   +0.17%     
=============================================
  Files             66       66              
  Lines           6162     6162              
=============================================
+ Hits            6114     6125      +11     
+ Misses            48       37      -11     

Impacted Files	Coverage Δ
include/ignition/math/Vector2.hh	100.00% <ø> (ø)
include/ignition/math/Vector3.hh	98.84% <ø> (+3.07%)	⬆️
include/ignition/math/Vector4.hh	95.57% <ø> (+1.10%)	⬆️

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[nkoenig]

@chapulina , do you want to try adding the nan types to the corresponding swig .i files?

[chapulina]

do you want to try adding the nan types to the corresponding swig .i files?

Done in 1233218

[azeey]

Looks good. Although I'd argue std::optional is more convenient than this for representing uninitialized values. For one, when checking if a variable is nan, you can't compare it against Vector::NaN directly. You'd have to use IsFinite, which I don't think would be immediately apparent to users. Also, IsFinite returns false if any of the members are nan, or inf, so it could potentially lead to bugs if Vector::NaN is used to represent uninitialized vectors.

[chapulina]

Thanks for the review, @azeey, I addressed your comments in 564de1c.

I also appreciate the feedback on the motivation for this PR. The specific use case that motivated this was a vector whose value keeps changing, and at times I actually needed to fill it with NaNs. But at other times I've also needed to "flag" that a vector's value was invalid temporarily, and I think it could be useful there as well, but I agree that there are multiple ways of going about this.