Allow instantiation of CmdStanModel with just an executable

[rok-cesnovar]

Summary

Fixes #488

With this we could do the following:

library(cmdstanr)

stan_file <- file.path(path.package("cmdstanr"), "logistic.stan")
# the current way
mod <- cmdstan_model(stan_file = stan_file)

new_loc <- "~/logistic"
file.copy(mod$exe_file(), new_loc)

# create if with exe file only
mod_exe <- cmdstan_model(exe_file = new_loc)
data_file <- file.path(path.package("cmdstanr"), "logistic.data.json")
mod_exe$sample(data = data_file)


# you can also specify both
# it will compile it to the exe_file location if compilation is needed
mod_exe <- cmdstan_model(stan_file = stan_file, exe_file = new_loc)
data_file <- file.path(path.package("cmdstanr"), "logistic.data.json")
mod_exe$sample(data = data_file)

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/)

[codecov-commenter]

Codecov Report

Merging #564 (9d203f4) into master (32ac2d5) will increase coverage by 0.03%.
The diff coverage is 98.00%.

@@            Coverage Diff             @@
##           master     #564      +/-   ##
==========================================
+ Coverage   93.07%   93.10%   +0.03%     
==========================================
  Files          12       12              
  Lines        3191     3206      +15     
==========================================
+ Hits         2970     2985      +15     
  Misses        221      221              

Impacted Files	Coverage Δ
R/model.R	93.15% <98.00%> (+0.19%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 32ac2d5...9d203f4. Read the comment docs.

[jgabry]

This looks great, thanks! I pushed a few minor doc edits and added a few very minor review comments/suggestions.

[rok-cesnovar]

Should be ready to review again.

[jgabry]

Looks good! Merging now