process_data() improvements

[rok-cesnovar]

Submission Checklist

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

Summary

Improves process_data in two ways:

• errors on missing data variables prior to starting the executable
• handles generating JSON for scalars or arrays/vectors of length 1 differently (fixes Length-one vectors are incorrectly read as single values #513)
• fixes Error reading matrix with large integral numbers #533 by forcing decimal points when outputing floating point values and applying as.integer() to R objects in the supplied named list that are supposed to be integers (thanks to @alyst)

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, Alexey Stukalov

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/)

[codecov-commenter]

Codecov Report

Merging #538 (c7c70e1) into master (58f0980) will decrease coverage by 1.50%.
The diff coverage is 100.00%.

❗ Current head c7c70e1 differs from pull request most recent head c08647c. Consider uploading reports for the commit c08647c to get more accurate results

@@            Coverage Diff             @@
##           master     #538      +/-   ##
==========================================
- Coverage   92.80%   91.29%   -1.51%     
==========================================
  Files          12       12              
  Lines        3071     3090      +19     
==========================================
- Hits         2850     2821      -29     
- Misses        221      269      +48     

Impacted Files	Coverage Δ
R/data.R	99.35% <100.00%> (+0.08%)	⬆️
R/model.R	90.88% <100.00%> (-1.06%)	⬇️
R/install.R	58.33% <0.00%> (-10.50%)	⬇️
R/utils.R	90.71% <0.00%> (-1.43%)	⬇️
R/csv.R	97.76% <0.00%> (-0.90%)	⬇️
R/run.R	95.23% <0.00%> (-0.50%)	⬇️

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[jgabry]

Looks good to me. Thanks!