Add sig_figs argument

[rok-cesnovar]

Fixes #313

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, Uni. of Ljubljana

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license the submitted work under the following licenses:

• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)

[codecov-io]

Codecov Report

Merging #327 into master will increase coverage by 0.00%.
The diff coverage is 92.85%.

@@           Coverage Diff           @@
##           master     #327   +/-   ##
=======================================
  Coverage   89.24%   89.24%           
=======================================
  Files          12       12           
  Lines        2538     2548   +10     
=======================================
+ Hits         2265     2274    +9     
- Misses        273      274    +1     

Impacted Files	Coverage Δ
R/args.R	99.54% <83.33%> (-0.23%)	⬇️
R/model.R	89.65% <100.00%> (+0.13%)	⬆️

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[jgabry]

This looks good, thanks! Optionally, maybe add a brief comment in the doc that increasing sig_figs increases the memory required?