Implement several renamings

[kbarros]

As described in versions.md:

Rename `polarize_spin!` to [`set_dipole!`](@ref) for consistency with
[`set_coherent!`](@ref). The behavior of the former function is unchanged: the
spin at a given site will still be polarized along the provided direction.

Rename `reshape_geometry` to [`reshape_supercell`](@ref), which is the
fundamental reshaping function. Rename `resize_periodically` to
[`resize_supercell`](@ref).

The constructor [`SpinInfo`](@ref) now requires a ``g``-factor or tensor as a
named argument.

[kbarros]

@ddahlbom and @Lazersmoke, there is not much to review in the implementation, but I added you as reviewers in case you have comments about these breaking changes.

In particular, making g a required argument to SpinInfo will impact a lot of code. I think it's the right thing to do, and @ddahlbom had suggested it, but I was hoping to get final confirmation before merging.

[kbarros]

David gave the thumbs up to merge.