RFC: move orditkplot to vegan3d

[jarioksa]

Arguments in favour of moving orditkplot from vegan to vegan3d:

• It is a special graphics function depending on tcltk and could quite well go with functions that depend on special packages such as rgl and scatterplot3d.
• Rumours say that tcltk may disappear from some platforms, and needs external libraries that may not exist everywhere. For vegan safety it could be removed.
• Removes 807 lines (634 lines of R, 173 lines manual) from vegan.

Stages of removal:

1. copy orditkplot files with history to vegan3d and release to CRAN.
2. Deprecate orditkplot.R in vegan, but keep code. Will be deprecated in the next release, but the code is duplicated and available in both packages. There is no need to hurry vegan release.
3. After the vegan release , make orditkplot defunct and remove its code in github (devel).

[jarioksa]

orditkplot is now in **vegan3d. See vegandevs/vegan3d#10.