Add UnitsNetSetup to hold global static state

[angularsen]

I built on your recent QuantityInfoLookup and added UnitsNetSetup to gather global state in a single place as a singleton, with the possibility of passing instances of it to static methods later to override the defaults.

This way, state is no longer scattered among multiple classes that depend on each other in non-obvious ways and where initialization order has caused issues up until now.

Changes

• Add UnitsNetSetup to hold global state as a singleton property Default that controls default state initialization.
  • Add properties for all "services" that depend on global state: UnitConverter, UnitAbbreviationsCache, UnitParser, QuantityParser
• Forward all other Default singleton properties from these services to UnitsNetSetup.Default and mark obsolete
• Add some TODOs where it seems functionality is missing

[angularsen]

Moved to angularsen#1267