[core] Added standard gravity and dBm Units

[Hilbrand]

Also did some clean up, which is in the first commit. The units are in the second commit.
Related to: https://github.com/openhab/openhab2-addons/pull/4913

[maggu2810]

What's the reason to make the classes final and its constructor private?
Do you see any critical if someone wants to create a sub class so we need to prevent it that strong?
Sure, it would make no sense (perhaps) but does it harm if?

[maggu2810]

I assume it has been wrong before but has been possible to use every "SmartHomeUnit" as "SIUnit" and "ImperialUnit".

        System.out.println(SmartHomeUnits.AMPERE);
        System.out.println(SIUnits.AMPERE);
        System.out.println(ImperialUnits.AMPERE);

It is not valid anymore.

It seems to be correct after the change because a SmartHomeUnit is not necessary an SIUnit.

But should we mark this as "API breaking" (and merge it) as it can break the consumers.

[Hilbrand]

The classes already had private constructors, except for SmartHomeUnits that had a protected constructor. Because of the private constructors is seems logical to make this more explicit and make the classes final. SmartHomeUnits was kind of misused by the other to classes. They extended SmartHomeUnits, but created a singleton of their own class, even while SmartHomeUnits also is used as a singleton class. So I changed the extend to the AbstractSystemOfUnits class they should extend (I think) and with that change SmartHomeUnits can have the private constructor too (and be a final class).

[Hilbrand]

I checked the openhab2 repo and found 2 issues of problems due to the changes, but only in tests. I've created a pr (5016) to fix them.

[maggu2810]

Thanks

[openhab-bot]

This pull request has been mentioned on openHAB Community. There might be relevant details there:

https://community.openhab.org/t/started-getting-compile-error-after-mvn-clean/69799/3