New option `org-time-clocksum-use-effort-durations'

New option `org-time-clocksum-use-effort-durations'

* org.el (org-time-clocksum-format): Add a version tag and add
to the 'org-clock group.
(org-time-clocksum-use-fractional): Ditto.
(org-time-clocksum-use-effort-durations): New option to allow
using `org-effort-durations' when computing clocksum durations.
(org-minutes-to-clocksum-string): Use the new option.

* org-clock.el (org-clocktable-write-default): Let-bind
`org-time-clocksum-use-effort-durations' to a new clocktable
parameter ":effort-durations".

Thanks to Sébastien Vauban for pointing the inconsistency.