Update R package structure

[pdiakumis]

In an effort to make PCGR a bit easier to maintain, update and test, I've started playing a bit with the R parts and think the pcgrr package setup can be improved with a few simple updates.

My strategy basically involves running the src/R/pcgrr R package through devtools::check() and systematically tackling the notes/warnings/errors. I'll split up these changes in separate PRs for slightly easier eyeballing, but currently I've got the following general changes somewhat done:

• move raw data generation script to data-raw/ as generally recommended by the R community
• add/update LICENSE files and DESCRIPTION
• @export all functions and allow NAMESPACE to be automatically updated by roxygen2
• update tidyverse functions to use the foo(.data$VAR, ...) syntax in place of foo(VAR, ...)
• prepare conda recipe for the R package alone
• decouple the 'heavy' BSgenome packages from the package requirements and instead move to the 'Suggests' field of DESCRIPTION

I'll be adding these changes to the dev branch so that we can also test without worrying about breaking stuff in master. In general I don't expect major functionality changes in PCGR itself, but maybe a variable here or there gets caught out somewhere leading to an error.

I'm keeping this issue here as the main point of communication.

[sigven]

Excellent, Peter! All for this.