Adds python interface to RollingMean, Color and Spline

[LolaSegura]

🎉 New feature

Goes on top of #216.

Related to #101 #210

Summary

Adds Python interface for three math classes: RollingMean, Color, Spline. For each class a python test has been created.

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)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

[codecov]

Codecov Report

Merging #218 (9e6df0f) into ign-math6 (b3804bf) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           ign-math6     #218   +/-   ##
==========================================
  Coverage      99.22%   99.22%           
==========================================
  Files             66       66           
  Lines           6162     6162           
==========================================
  Hits            6114     6114           
  Misses            48       48           

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[LolaSegura]

General comments

Given that there is no assignment in python the operator=() is ignored by swig. So the only way left to copy elements of the custom classes is by using the copy constructor.

The operator[]() was being ignored as well and this warning was shown when trying to build the ignition-math6 module Warning 389: operator[] ignored (consider using %extend). So as suggested to implement the indexation method we decided to create an extension.

The operator<<() had to be implemented using an %extend modifier as well. Also, for it to work correctly the std_string.i file had to be included.

Color

In C++ the getter methods are overloaded, so for example, getting the r element of a color could be done with both of this methods:
float R() const;
float &R();
But when creating the python interface swig interpreted this as if one method was shadowing the other. Given that a setter method is defined as well (void R(const float _r)) the decision was to implement only the first one.

[francocipollone]

@LolaSegura Please create another PR with the files related to Matrix3, Matrix4, Pose3, and Quaternion in order
to avoid this PR to grow this much and make it easier to review.

[francocipollone]

Nice! Just a few comments

[francocipollone]

nit: Add an extra empty line at the end

[LolaSegura]

Addressed

[francocipollone]

LGTM!

[francocipollone]

@scpeters @chapulina It is ready for an extra review!

[scpeters]

I just resolved conflicts after merging #220