mod$variables works w includes in precompile state (fix #680)

[MKyhos]

Naive attempt to tackle #680.

Description

Approach
Adapt the CmdStanModel$variables() query method in such a way that it returns either private$include_paths_ or private$precompile_include_paths_.

Alternative that was considered:
During call to model_variables(), make the value passed to the include_paths argument dependent on the state of the model. I.e., if no executable is present, insert private$precompile_include_paths_, and otherwise self$include_paths().

Concerns: it seems somewhat not optimal to query the state of the model (compiled / not compiled) via the existence of the executable, but I saw this approach also elsewhere in the code.

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Submission Checklist

• Run unit tests
• Add unit tests for added / fixed behavior.
• Declare copyright holder and agree to license (see below)

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Malte Kyhos

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[rok-cesnovar]

Thank you @MKyhos. Congrats on your first merged contribution to cmdstanr 🎉 🎉 ! We appreciate it!

[MKyhos]

Thank you @rok-cesnovar :) I was wondering, maybe an active binding for the state of a model (compiled / not compiled) would be beneficial? It would hide the logic, given that there could be even more sophisticated checks that do not fit well into a condition.

[rok-cesnovar]

maybe an active binding for the state of a model (compiled / not compiled) would be beneficial?

Agreed, though this PR is good to go as is. Maybe if you have time, a refactor is always welcome.

[MKyhos]

Cool :) Then merge whenever suits you, I will not add something on this branch / PR 👍