Umd3 feature/ renamed into features/

As discussed in template-library-standard#22, umd-3 feature/ should have been features/ to be compliant with what is expected by Aquilon and to be consistent with what is done in template-library-standard.

Would be nice to have this PR merged before the 14.4 release. Impact should be minimal on sites as these templates are generelly referenced mainly (if not only) from other umd-3 templates. And it is easy to make the necessary changes with a couple of sed commands....