Month-week and quarter-week datetrees (RFC and package announcement)

[Jack Kamm]

I have started a package that defines org-capture datetrees following
year-quarter-week and year-month-week formats:

https://gitlab.com/jackkamm/org-grouped-weektree

I'd appreciate feedback on 2 points:

1. Are any of these datetree formats worth upstreaming into org-mode
   proper?

2. Can we add a public interface for `org-datetree--find-create', and
   are there any suggestions on how to do it?

Regarding #2, an ugly aspect of my current implementation is the abuse
of the private function `org-datetree--find-create'. I also pass in
the week or quarter for the DAY and MONTH arguments of this function,
though I note that `org-datetree-find-iso-week-create' does something
similar in its implementation.