The const_cast in Deck::get*UnitSystem are potentially undefined
behaviour under a series of (plausible) conditions, and are deprecated
in favour of mutable members. Removes initUnitSystem from the public
interface, as the initialisation is handled on the first getUnitSystem
anyway, cleaning up the Deck interface slightly.
Adds proper constness to the use of unit systems that don't actually
modify the internal state in any way.
135f405e93