Feature/issue 68 read stan csv

I changed the R code for `read_stan_csv()` so it is able to continue if it can't find the "Elapsed time" comments at the end of the file. There is also a unit test in place to test that this works.

This means partial runs can be read through the R interface.

@maverickg or @bgoodri, want to double-check the updated functionality before merging?