Return to using original VMTK instead of fork?

[JostMigenda]

We are temporarily using a forked version of VMTK; see CemrgApp/CMakeExternals/VMTK.cmake.
However, it looks like the main repo of VMTK is somewhat active again. Notably, there appears to have been a release v1.5.0 which upgraded the VTK/ITK dependencies; it’s also available under that version on conda-forge, though there’s no tag/release on the GitHub repo.

Since it may not be a good use of our resources to maintain our own fork of VMTK indefinitely, we should check whether we can return to using the official VMTK release (and upstream any essential changes we’ve made).