Consider using R.cache for `dir`

[brry]

Consider whether to use R.cache for dir management. https://stat.ethz.ch/pipermail/r-package-devel/2019q4/004783.html

Open questions:

• implementation details
• breakage of user code depending on the current default "dir=DWDdata"
• usage of getwd()

[brry]

Declined for the following reasons:

• dirDWD already handles things quite nicely
• I assume most usages of rdwd are within a project anyways, where a local DWDdata folder makes sense
• it would be a lot of work on my side
• existing code might break for many users

Unless someone brings this up again with convincing arguments for R.cache, I'll consider changes unnecessary.