org.el: use `user-error' instead of `error'.

* lisp/org.el (org-next-block, org-reftex-citation)
(org-create-math-formula, org-check-agenda-file)
(org-occur-next-match, org-auto-repeat-maybe)
(org-offer-links-in-entry, org-open-at-point)
(org-clone-subtree-with-time-shift):
Use `user-error' instead of `error'.
ob-core.el: Replace a use of remove with remq

* lisp/ob-core.el (org-babel-del-hlines): Use `remq' for `remove'
Replace org-babel-*-graphical-output-file with generic version

* lisp/ob-core.el (org-babel-graphical-output-file): New generic function.
* lisp/ob-R.el (org-babel-R-graphical-output-file): Delete specific version.
(org-babel-expand-body:R): Use generic version.
* lisp/ob-maxima.el (org-babel-maxima-graphical-output-file): Delete
specific version.
(org-babel-maxima-expand): Use generic version.
* lisp/ob-octave.el (org-babel-octave-graphical-output-file): Delete
specific version
(org-babel-execute:octave): Use generic version.
org-agenda.el (org-agenda-finalize): Filter by top headline

* org-agenda.el (org-agenda-finalize): Filter by top headline
if `org-agenda-top-headline-filter' is set.

Thanks to Brady Trainor for reporting this.
Merge branch 'maint'
ox-icalendar.el (org-icalendar-use-scheduled): Fix typo

* ox-icalendar.el (org-icalendar-use-scheduled): Fix typo.
Merge branch 'master' of orgmode.org:org-mode
Fix typos

* org.el (org-get-agenda-file-buffer): Fix docstring.

* ox-icalendar.el (org-icalendar-use-deadline): Fix typo in docstring.