Add create_cutting_test() and multitest()

[jdblischak]

This PR introduces a new function create_cutting_test(). Similar to create_cutting(), it is a function factory that generates functions that can easily be passed around. This enables us to do various new things.

1. I created a new function multitest(). It allows running multiple tests at once on a cut data set. It currently returns a list because the test functions do not return consistent outputs
2. I updated the sim_gs_n() argument test to accept a list of test functions, one per cutting. This is mostly a proof of concept at the moment. More work will be required before we can support allowing multiple tests per cutting and mixing heterogenous tests (as it is currently written sim_gs_n() requires all the test results to be combined into a single output data frame)

I also took the time to clean up some of the roxygen2 docs since I was the one who introduced a lot of the macros.

Lastly, I didn't touch maxcombo() in this PR, but you can imagine that we could also use create_cutting_test() to be able to pass any arbitrary number of test functions to maxcombo(). But just like for sim_gs_n() and multitest(), this would be facilitated by the test functions having consistent output.

[jdblischak]

Rebased onto 9ac40f3 to include #214

[nanxstats]

LGTM