Move ROOT to the pushforward/pullback model

[vgvassilev]

Here, we need to move https://github.com/root-project/root/blob/master/math/mathcore/inc/Math/CladDerivator.h to follow what has been done in Clad: https://github.com/vgvassilev/clad/blob/master/include/clad/Differentiator/BuiltinDerivatives.h where all functions are based on pushforwards and pullbacks.

[arora-vidushi]

Hello Vassil,

To work on this issue, I have some doubts that I would like more information upon:

++ Does the root use CLAD latest version and has tests to check custom derivative?
++ Also, if we change the custom derivative function in Root, will it break the existing tests?

Thanks,
Vidushi

[vgvassilev]

Hello Vassil,

To work on this issue, I have some doubts that I would like more information upon:

++ Does the root use CLAD latest version and has tests to check custom derivative?

Yes, it uses the last version of clad (v1.0).

++ Also, if we change the custom derivative function in Root, will it break the existing tests?

No, and that's the issue, this part has no test coverage in ROOT but I think we should develop some basic tests after we migrate.

[vgvassilev]

This issue is independent on the release of Clad and thus not a release blocker.

[vgvassilev]

This has been resolved. Thanks a lot @arora-vidushi!