WIP: More cleanup - Series and Stats naming and documentation

I update F# Formatting and added github source links to the generated documentation &
did various tweaks in the Series and Stats module documentation (there are still pieces
missing, but it looks pretty good and well organized now).

I finished renaming (#179) and renamed `Series.union` and `Series.append` to
`Series.merge`. I also used `mergeAll` instead of `mergeN`, because I think the
name "all" appears in multiple places of the API (so it is more consistent than
using "N").

Finally, I moved two last statistics-related functions `Series.minBy` and `maxBy` to
the `Stats` module. I also deleted the obsolete functions - we probably want to handle
compatibility better after the next release, but since we are renaming so many things,
I think there is no point having _some_ obsolete functions but not _all_.
Do more renaming ("series" and "column") (c.f. #189)

I found a few more occurrences of "series" and replaced them with
"column" to finish the idea in PR #190.

There was another minor inconsistency where we sometimes used
`cols` and sometimes `columns` in the F# API, so this changes
everything (in the F# module only) to use `cols`. I would be equally
happy with `columns` though (I just picked the more common one).