Add eigen tensor conversions

[jgbradley1]

This PR addresses issue #17237. The primary contributions are two conversion functions (cv2eigen and eigen2cv) for the Eigen::Tensor module along with unit tests.

Pull Request Readiness Checklist

• I agree to contribute to the project under OpenCV (BSD) License.
• To the best of my knowledge, the proposed patch is not based on a code under GPL or other license that is incompatible with OpenCV
• The PR is proposed to proper branch
• There is reference to original bug report and related work
• There is accuracy test, performance test and test data in opencv_extra repository, if applicable
  Patch to opencv_extra has the same branch name.
• The feature is well documented and sample code can be built with the project CMake

[jgbradley1]

I added a third convenience function (cv2eigen_tensormap) which provides a Tensor interface to cv Mat data without copying any data.

[alalek]

Type check is required here through CV_CheckTypeEQ(mat.type(), traits::Type<_Tp>::value, "")

[jgbradley1]

The suggestion almost worked. I had to modify to account for multiple channels.

CV_CheckTypeEQ(mat.type(), CV_MAKETYPE(traits::Type<_Tp>::value, mat.channels()), "");

[alalek]

@jgbradley1 You are right!

Perhaps this one should be clear:
CV_CheckDepthEQ(mat.depth(), traits::Depth<_Tp>::value, "");

[jgbradley1]

Yes, checking the depth would have been clear (and less verbose code). It doesn’t matter to me which approach we use. Since you already approved the merge, I’ll let you decide if it’s worth opening up another PR to update the code.

[alalek]

Well done! Thank you 👍