Add warning message for transformed data

[sunxd3]

"transformed data" are variables that can be computed with data.

In JuliaBUGS, these values are computed during compilation. And these variables will not end up in the final graph.

For standalone language like BUGS and Stan, it make sense to add these into the model definition. But for JuliaBUGS, it makes more sense to compute these data transformations in Julia.

This PR add some warning message for this.

[coveralls]

Pull Request Test Coverage Report for Build 11091905450

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 6 of 6 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.04%) to 83.098%

---

Totals
Change from base Build 11091696578:	0.04%
Covered Lines:	1470
Relevant Lines:	1769

---

💛 - Coveralls