Allow different lattice spacing in each dimension for `kwant.continuum`

Generalize the lattices in kwant.continuum to rectangular lattices, in 2 dimensions or to a lattices made of rectangular cuboids, in 3 dimensions.

The parameter grid, can now be a scalar, for which all lattice spacings are equal, it can be a sequence of scalars, for which each scalar is the lattice spacing in the appropriate dimension, and it can be a kwant.lattice.Monatomic, that can be a 1, 2, or 3D lattice.

In the latter case, grid should be a lattice with diagonal primitive vectors, but this is not enforced and does not raise any errors. Although, the diagonal components will be used as primitive vectors magnitudes in the discretization.

Tests for this feature are yet to be written, and previous tests should pass with no modification.

Fixes #197 (closed)

Ok, one test failed. The test that asserts that the cases this MR fixes, should not pass:)

@pablopiskunow Thanks! I will try to help finalising this MR. This should definitely aid at least few simulation cases before the non-uniform meshes will come to exist.

good, thanks to you for the help!

@r-j-skolasinski what is the status of this MR?

Sorry, didn't have time yet to look due to the chaos around defence. It is WIP then still. I will take a look soon.

How can I help with this MR? Shall I be making PR to Pablo's fork or @pablopiskunow can give me write access to his fork?

I can give you access

Thanks!

mentioned in merge request !225 (merged)

added 24 commits

• 28e1e3ee...19c7e943 - 23 commits from branch kwant:master
• 94169791 - allow different lattice spacing in each dimension

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added 3 commits

• be7790ac - read properly position of sites from offseted grid, closes #199 (closed)
• e2e8594b - allow different lattice spacing in each dimension
• aa13b7d3 - fix rendering of symbolic discretization + fix existing tests

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added 1 commit

• 59eeb6a8 - simplify handling different input types of "grid"

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@pablopiskunow I fixed few minor things:

• I rebased on top of issue_199 - it changes logic of reading coordinate to use .pos method of sites
• Fixed rendering of symbolic discretization
• cleaned part when different types of grid input are handled (scalar, seq. of scalar, ...)
• adjust existing tests and fixed checking lattice constraints (together with above point)

I think we are only missing a proper tests for new functionality. Once we add those and squash it should be ready for merge.

added 1 commit

• 646ba2c8 - reformat docstring to make Sphinx happy

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added 3 commits

• 646ba2c8...60f02546 - 2 commits from branch kwant:master
• aa883200 - add support for rectangular discretization grids

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I rebased on kwant/master and squashed commits.

@pablopiskunow You can remove WIP to signal that it is ready for review by others. I will add tests for new functionality now.

unmarked as a Work In Progress

changed title from {-WIP: in kwant.continuum allow different lattice spacing in each dimension-} to {+Allow different lattice spacing in each dimension for kwant.continuum+}