Generic quantities update

[ebfortin]

I've been off the development on UnitsNet for some time. Was there any progress made on the use of generic?

I implemented a double double type, giving 128-bits like float, which would be quite interesting to use with UnitsNet. But without generic quantities, it would be a pain to implement.

[ebfortin]

I also did some tests with generic operators. Not quite performing enough yet, but getting there.

[tmilnthorp]

I had made #714 to try it out some but haven't got a chance to go back. We merged #698 yesterday that allows math operations on generic types.

[stale]

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