Implement new `U' mode switch for table formulas to omit seconds in durations
* lisp/org-table.el (org-table-duration-custom-format): Add new
HH:MM format.
(org-table-duration-hour-zero-padding): New option.
(org-table-eval-formula): Select second-less format if
requested.
(org-table-time-seconds-to-string): Implement formats without
seconds and without zero-padding for hours.
* testing/lisp/test-org-table.el (test-org-table/duration):
New test for second-less durations.
* doc/org.texi (Formula syntax for Calc)
(Durations and time values): Document the U mode switch.