Consider updating fread call to not return integer64

[kylebaron]

readr doesn't do it and I think most of the time, I'd rather have double than integer64.

https://github.com/metrumresearchgroup/bbr/blob/main/R/nm-file.R#L142-L147

[seth127]

Thanks for this Kyle. For context, some downstream packages essentially treat integers as factors in some cases, and this change would address that issue.

[seth127]

Some added detail (that Kyle shared offline), as he notes, readr stopped doing "integer guessing" in readr 1.2.0 (released in 2018).