Limit pint unit systems that are automatically considered

[euronion]

Related to the issues in #51 , where ton was accidentally interpreted as US_ton:

Is it possible in pint to limit the unit systems that are available and considered?

ureg.default_systems = "SI" does not unregister the "US" and "imperial" system, it just changes the default:

ureg.default_system = "SI"

ureg.ton.systems
> frozenset({'SI', 'US', 'imperial'})

a) If it is possible, at least document it
b) Consider choosing "SI" / "mks" as exclusive default systems, with "Imperial" and other systems optionally to be enabled?

FYI @finozzifa : I opened an issue after our discussion to keep track of it, but no action needed on this particular issue atm.