Add MeshPotential calculator

[kvhuguenin]

Add a first working version containing:

1. the class MeshPotential that computes the electrostatic potential for one or an entire list of atomic structures at the location of each atom
2. two auxilary classes (MeshInterpolator, FourierSpaceConvolution) that each perform the two key subtasks

The code comes with

1. unit tests covering most relevant use cases of the currently implemented functionality
2. sphinx documentation of all external and internal functions
3. torchscript compatibility

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📚 Documentation preview 📚: https://meshlode--3.org.readthedocs.build/en/3/

[PicoCentauri]

Some first comments on the docs. I will continue later with more detailed. comments.

[PicoCentauri]

Looks very clean and documentated. I have some comments basically that will help users if the provide wrong shapes of the arrays.

Also, maybe we should also do examples for the FourierSpaceConvolution and the MeshInterpolator classes. What do you think?

[kvhuguenin]

I have now implemented all the changes that you suggested, apart from keeping the test_calculators.py file the way it is. I have tried to go through all the main files again to frequently check that the shapes of the provided tensors are correct and added the `` marks for more readable documentation, with an example file for how to use MeshLODE in examples/ .

[ceriottm]

I added an example of MeshInterpolator and FourierConvolution. As far as I'm concerned, it's good to go (hoping that tests pass...)

[PicoCentauri]

Hmm I am a bit confused by the failure of the jobs. We haven't touched the files and the GH action service seems to run fine.