Flat list output

Convenience for situations where you'd like to output list content (eg, choices of models) as a single-line of output - as also discussed with @andy earlier this week.

marked as a Work In Progress

... just adjusting methods names and default parameters as discussed with @andy

added 7 commits

• 8ed9f029...d0732a24 - 3 commits from branch develop
• f56390a8 - STYLE: make UList::writeEntry(Ostream&) protected
• 3fcc47e6 - STYLE: minor code adjustments
• 9a092319 - ENH: support ASCII List output on a single-line
• 3dec91f6 - STYLE: use flatOutput when reporting known file-extensions

Compare with previous version

unmarked as a Work In Progress

merged

mentioned in commit f60ba5b0