Clean up factor range and add factor method

[bryevdv]

This is preparation for working on #601. In order to implement that we need and inverse method for synthetic that returns factors, given a synthetic coordinate as input. Looking at adding that, it became clear that factor_range.ts was long overdue for serious cleanup.

This PR breaks up the parts of factor_range.ts in to much smaller functions and methods, adds more tests for these where possible, and also adds a factor method to do the inverse synthetic calculation.

All of the original unit tests were maintained, modulo trivial cosmetic changes for some renames, etc.

I expect map_one_level, etc could also be cleaned up. But since they aren't too terrible I decided to avoid more change risk than necessary.

Edit: examples report checked, everything looks correct at a glance.

[bryevdv]

@mattpap suddenly seeing this locally (and I guess in CI) after merging latest branch-3.6

Running in Chrome/127.0.6533.120 using devtools protocol 1.3
warning: 127.0.6533.120 is not supported; officially supported version is 118.0.5993.88
internal error: failed to collect tests: ReferenceError: Tests is not defined
    at <anonymous>:1:1
[14:43:33] Finished 'test:unit' after 1.12 s

any ideas?

Edit: I guess it's the last commit for the internal properties but the error message does not give much to go on

Edit2: missed the setv with "levels" at the end of _init

[bryevdv]

@mattpap next issue.. I am not seeing any eslint issues locally, even though AFAICT I have exactly the same version installed as CI:

dev312 ❯ npm list |grep lint
│ ├── @types/eslint@8.56.7
│ ├── eslint@8.57.0 deduped
│ ├── typescript-eslint@7.16.0
├── eslint@8.57.0

Edit: dunno why the local/CI diff but it's better as a switch in any case

[bryevdv]

OK I embarked on a (slightly) more ambitious refactoring. It bothered me that we had to switch on the levels in multiple various places, so I encapsulated everything for each level in a subclass of a generic FactorMapping. This also makes it more explicit how it is ok for an L2 mapper to map L1 factors, etc. and also hides all the "boxed" handling purely internally. I still did not alter map_one_level, etc except to tighten up their types, in order to keep the bulk of existing test as-is.

Edit: went through examples report in rather fine detail and everything looks correct there as well.

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