Generate operators from unit relations defined in JSON

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Related #1200

In the PR adding generic math (#1164) @AndreasLeeb states:

Regarding the operators in the *.extra.cs files, that could be tackled easily by describing the dependencies (operations) between different quantities in the quantity JSON files, and then the operator overloads and the generic math interfaces for the quantity structs could also be automatically generated. But that's a topic for another time 😄

I decided to give this a shot.

UnitRelations.json contains relations extracted from the existing *.extra.cs files. I decided on a new file because multiplication is commutative and I didn't want to duplicate these in the individual quantity JSON files, or risk missing one or the other, so it's best to define them once in one place. The generator handles this by generating two operators for a single multiplication relation.

The relations format uses the quantities method names. This is a bit unfortunate, but it's the best I could come up with without making the CodeGen project depend on UnitsNet, which would create a bit of a chicken/egg problem. This is not unheard of (self-hosted compilers) but I wanted to keep it simple for now.

The generated code enables the removal of 44 *.extra.cs files, and the 17 remaining contain much less code.

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@angularsen What's your opinion about this? Is this a worthwhile endeavor in the right direction?

[angularsen]

Apologies, I thought this was still being worked on so I haven't looked at it yet.
The PR description sounds very useful, I'm just curious if there are any challenges to this approach or not.

Let me read more through the code and get back to you 👍 Will try to find time the next couple of days.

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No worries, I should have been more clear that this is a proposal in need of feedback. God jul! 😄

[angularsen]

I think this looks very promising and a big improvement on the existing approach.

A few ideas below.

[angularsen]

I think this looks solid! This is some impressive work 👏

Some minor suggestions for you to consider.

The biggest one for me is to get some peace of mind that we are not breaking anyone as it is hard to verify by review.

I added a suggestion to maybe use reflection to compare before/after of all quantities and their public members, by outputting to text and diffing that.