Fix parsing ambiguous abbreviations when lowercase

[angularsen]

Fixes #590

• Add separate abbreviation-to-unit mapping that preserves case sensitivity
• Fall back to case sensitive mapping if more than 1 unit is found
• Add test

I'm sure we can optimize this with a single map, but I don't think we're talking any significant memory footprint here.

[tmilnthorp]

Don't know how this even slipped my mind. Yuck. I think it might be better to just revert the whole ignore case PR. Ambiguous between yotta/yocto, zetta/zepto, peta/pico, and mega/milli prefixes. Seems a bit silly to have to always specify case sensitivity for prefixed units, but not for others just in case case doesn't matter (if you even know it's a prefix, what if it's from user input?).

[angularsen]

Yeah, it feels yucky to have missed this and I also have second thoughts on the whole case insensitive support.

I think it's nice that you can parse both 5 PSI and 5 psi for free, without having to specify both abbreviations. On the other hand, adding some entries in JSON is a one-time effort and also not much work. Also, then we don't allow weird things like 5 PsI, which could potentially mean something else entirely and then they would get the wrong unit instead of UnitNotFoundException.

I'm a bit torn, on one hand we are in a good position to make it super easy to parse user input without being too strict on perfect casing that I think many often get wrong. It's hard to do this outside the library.

On the other hand, it feels a bit hacky, but in the implementation at least ensures there is no ambiguity by falling back to case sensitivity matching if more than one unit matches.

The only problem I can think of is when trying to parse a unit we haven't added yet, that happens to be identical to an existing unit with different casing and instead of getting UnitNotFoundException you get the wrong unit.

To counter this, I propose to make parsing methods case sensitive by default, with a new parameter ignoreCase. That should give us the best of both worlds, allowing users to explicitly parse with ignore case?

[angularsen]

I'm merging this now, just pushed a new nuget out and I don't want this bug in it. We can revise this later.

[angularsen]

https://github.com/angularsen/UnitsNet/releases/tag/UnitsNet%2F4.5.1