Add parsers for cpp opts

[katrinabrock]

Also

• Move some helper functions out of model.R into new file
• Add tests for newly added functions

Submission Checklist

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

Summary

• Move a few cpp option related functions into their own file (these are unchanged)
• Add new parser functions and tests (several of these functions are not yet used outside of tests, but will be used with new cpp option logic)

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):
Max Planck Institute of Animal Behavior

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/)

[SteveBronder]

This looks great! Few little things that are mostly just cleanup, but overall good

[SteveBronder]

Suggested change

	for (i in seq_along(cpp_options)) {
	option_name <- names(cpp_options)[i]
	cpp_option_names = names(cpp_options)
	for (i in seq_along(cpp_options)) {
	option_name <- cpp_option_names[i]

[katrinabrock]

@jgabry can you merge this one also?

[jgabry]

Merging now. Thanks @katrinabrock! And thanks @SteveBronder for reviewing.