Rename fit$metadata()$model_params to fit$metadata()$variables

[rok-cesnovar]

Submission Checklist

• Run unit tests
• Declare copyright holder and agree to license (see below)

Summary

Fixes #504

This keeps the old $model_params as well but then remove it for the 1.0 release.

Copyright and Licensing

Please list the copyright holder for the work you are submitting
(this will be you or your assignee, such as a university or company):
Rok Češnovar

By submitting this pull request, the copyright holder is agreeing to
license the submitted work under the following licenses:

• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
• Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)

[rok-cesnovar]

This is just a rename so if the test pass I will just merge.

[codecov-commenter]

Codecov Report

Merging #545 (5982d28) into master (ba38490) will decrease coverage by 1.53%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #545      +/-   ##
==========================================
- Coverage   92.81%   91.28%   -1.54%     
==========================================
  Files          12       12              
  Lines        3119     3120       +1     
==========================================
- Hits         2895     2848      -47     
- Misses        224      272      +48     

Impacted Files	Coverage Δ
R/csv.R	97.76% <100.00%> (-0.89%)	⬇️
R/fit.R	98.22% <100.00%> (ø)
R/install.R	58.33% <0.00%> (-10.50%)	⬇️
R/utils.R	88.88% <0.00%> (-1.39%)	⬇️
R/model.R	91.00% <0.00%> (-1.05%)	⬇️
R/run.R	95.23% <0.00%> (-0.50%)	⬇️

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[jgabry]

Thanks!