Create cell class and remove ase

[jimnel]

This resolves #12

• Creates a new cell class that replaces ase.Cell.
• With this ase is removed from qse.

[sherryblair]

I think it looks good overall, I added one suggestion

[sherryblair]

maybe we should add an error message incase the matrix has no inverse?

if self.rank < 3:
raise ValueError("Reciprocal lattice undefined for rank < 3.")

[rajarshitiwari]

The Cell class I feel needs a bit more work before we merge. @jimnel Please look at the methods that ase.Cell offeres. While there are several methods that we may not need, there are some that I think we should have have.

Also, while we are changing it. We need to implement it in a way that removes one drawback that original cell object had, and that was forcing 3D structure. Let's give it some thought to see if we can make it a bit flexible for 1D, 2D, 3D. If it turns out hectic, we back out and keep 3D.

A point to note: the cell.rank attribute in original ase.Cell doesn't really compute the rank of the 3D matrix, but rather computes the number of non-zero lattice vectors, and it also uses it to get reciprocal in reduced dimension.

[rajarshitiwari]

I think there should be some kind of mask to do reciprocal in reduced dimension. See the original ase.Cell class for reference.

[rajarshitiwari]

@jimnel Check this snippet from original ase.Cell for reciprocal

def reciprocal(self) -> 'Cell':
        """Get reciprocal lattice as a Cell object.

        The reciprocal cell is defined such that

            cell.reciprocal() @ cell.T == np.diag(cell.mask())

        within machine precision.

        Does not include factor of 2 pi."""
        icell = Cell(np.linalg.pinv(self).transpose())
        icell[~self.mask()] = 0.0  # type: ignore[index]
        return icell

[jimnel]

@rajarshitiwari
We can discuss this on Friday. I've a few ideas.