New check: only use *.R and not *.r file extension

[Bisaloo]

As per https://cran.r-project.org/doc/manuals/r-release/R-exts.html#Package-subdirectories

[Bisaloo]

I'm ready to submit a PR for this. Would you be interested? More generally, what's the status of this package? Is it still under active development?

[dougmet]

This sounds good. Will check with the main maintainers.

And yes, it’s actively maintained

[dougmet]

I think this would be a good PR. I tried to check if CMD check picks it up (of course on two case-insensitive file systems) but I'm pretty sure it doesn't.