Track spacegroup setting

[kbarros]

Keep track of the affine transformation data $(P, p)$ that defines the spacegroup setting. This transformation can be used to map basis vectors $(a, b, c)$ in any custom setting to the "standard" ITA setting via: $[a_s b_s c_s] = [a \, b \, c] P^{-1} $. It can also be used to map positions in a custom setting coordinates $x$ to standard setting coordinates via: $x_s = P x + p$.

With this setting information, it becomes possible to look up Wyckoffs on the fly, using tables that have been pretabulated for the ITA standard spacegroup settings. It also becomes possible to make use of other packages, such as Brillouin.jl, that require basis vectors in a standard setting.

Fixes #44.