Implemented add handler in BaseVectorField and Differential

[davidepittet]

References to other Issues or PRs

Partially fixes #26478: this issue has multiple steps to be taken, this addresses the first one.

Brief description of what is fixed or changed

A __add__ method is implemented in both BaseVectorField and Differential classes, that checks type constraint before delegating sum over to Add class.

Other comments

I would like a feedback on the way I implemented this change. In particular, I am uncertain whether to follow this path with the subsequent steps or write a new class to handle tensor product sum, similarly to MatrixExpr and MatrixAdd.

Release Notes

• diffgeom
  • Implemented __add__ method for BaseVectorField and Differential

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• diffgeom
  • Implemented __add__ method for BaseVectorField and Differential (#26499 by @davidepittet)

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#### References to other Issues or PRs
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Partially fixes #26478: this issue has multiple steps to be taken, this addresses the first one.

#### Brief description of what is fixed or changed
A \_\_add__ method is implemented in both BaseVectorField and Differential classes, that checks type constraint before delegating sum over to Add class.

#### Other comments
I would like a feedback on the way I implemented this change. In particular, I am uncertain whether to follow this path with the subsequent steps or write a new class to handle tensor product sum, similarly to MatrixExpr and MatrixAdd.

#### Release Notes

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* physics.units
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<!-- BEGIN RELEASE NOTES -->
* diffgeom
  * Implemented \_\_add__ method for BaseVectorField and Differential
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