Making the methods from UnitMath that exist in System.Linq as [Obsolete]

[lipchev]

Making the Min / Max of an IEnumerable and their selector overloads [Obsolete] in v5 (to be removed in v6).

[lipchev]

@angularsen Note that the extensions in System.Linq aren't directly applicable to a collection of IQuantity as it doesn't implement the IComparable interface (and even if it did- the result of sorting a mix of different quantities wouldn't be obvious).

If you agree that we don't want them any more, I can simply remove these in #1587 and we'll have less to talk about next time..

[angularsen]

Yes, let's remove Min/Max extension methods on the untyped IQuantity. It doesn't make sense to compare Mass and Length.