Remove the external/dmlt directory

[robertoostenveld]

It is not maintained by the original developer, not actively maintained by us, and its use is not promoted through documentation. See also fieldtrip/website#594

[robertoostenveld]

as external users might still have a use for it, we can consider branching the external/dmlt directory off into a separate repository on github.

[robertoostenveld]

I think we can use https://docs.github.com/en/get-started/using-git/splitting-a-subfolder-out-into-a-new-repository or https://amandawalkerbrubaker.medium.com/splitting-your-git-repo-while-maintaining-commit-history-35b9f4597514 to keep the history consistent and clean.

[robertoostenveld]

An old copy of the toolbox is available on https://github.com/bahramisharif/DMLT. The original copy was on https://github.com/marcelge but that does not have any public repositories any more. It seems that Marcel has moved to a new account https://github.com/marcelvangerven, although there are no public repositories there either.

There were some commits to external/dmlt in 2019 from @schoffelen and from @matsvanes, I think we should not loose them.

[robertoostenveld]

I have copied the code to a public archived repository at https://github.com/fieldtrip/dmlt