Allow output(x) <- TRUE syntax

[richfitz]

This PR restores support for odin1's old syntax for outputting a variable that was used in an intermediate calculation. We can write:

x <- a + 2
output(x) <- TRUE

and also use x within calculations in the graph This was very fiddly to get right in the end, but the overall implementation seems ok. Doubtless there will be corner cases to catch.

This only affects ODE models so won't affect many people, but is required to support the deterministic malaria model, which is my target to support for replacing odin1

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (7fc9c2c) to head (ffd4a45).
Report is 11 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #136   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           20        20           
  Lines         3504      3546   +42     
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+ Hits          3504      3546   +42     

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