Fixing up R CMD Check

[tonyk7440]

Following up on #34 to try to get this to build successfully but have hit a bit of a roadblock.

1. Error from logs

E  Running 'testthat.R' [70s] (1m 9.8s)
   Running the tests in 'tests/testthat.R' failed.
   Complete output:
     > library(testthat)
     > library(treesnip)
     Loading required package: parsnip
     > test_check("treesnip")

which doesn't tell me much.

I then tried running devtools::test() next but my R session crashes each time.

I'm not sure if this is a 'my machine' issue, does this happen for others?

2. Other than that, I've updated an arg to use sample_prop instead of sample_size so that an error doesn't get generated. I think sample_size should be substituted for sample_prop in general to avoid confusion, does anyone have an opinion on this?