Proof-of-concept data/compare in odin

This PR provides parse support for the new compare syntax (see https://github.com/mrc-ide/odin.dust/issues/130 and the corresponding PR https://github.com/mrc-ide/odin.dust/issues/131). I doubt we'll ever support this in O.G. odin, but we will eventually in the js version which uses a dust-like target.

Here, we just try and wrangle the input and do as little validation as possible. One nice thing about this approach is that all the bits for checking unused variables etc come along for free.

Note that this is intentionally undocumented; we'll document it properly in odin.dust when supported, and it really will just represent a ghost feature in odin

Alex; just adding you here as a FYI, but any comments very welcome

A reminder of the order of PRs:

compare/data definition within the odin DSL (you are here)
generate code from this in odin dust (draft PR linked above)
new support in dust for models that can produce their own derivatives
generate code using this compare dsl to target this new dust support

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (938cb5d) 100.00% compared to head (e069323) 100.00%.

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Additional details and impacted files

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mrc-ide / odin

Proof-of-concept data/compare in odin #294

Codecov Report