Feature change: consistent supply of date formats

[martinctc]

Currently, functions such as IV_by_period() require a supply of dates (but character variables) in a particular Y/M/D format. It would be helpful to standardize this across the package, and also enable the direct supply of Date class variables where formats do not need to match.

[martinctc]

The cranlogs main function uses the following input format:

Start date, in yyyy-mm-dd format, or last-day.

https://r-hub.github.io/cranlogs/reference/cran_downloads.html