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)