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.

[kbarros]

This PR also changes the Crystal constructor to employ the ITA standard setting by default. This means that in most cases the user will only need to provide the a space group number. The setting option gets renamed choice for consistency with spglib, but in most cases it will no longer be needed.