Annif

This is a bugfix/optimization release that improves evaluation functionality.

* 236 / 238 Speed up optimize command and avoid high memory usage

New in this release:

* 218 simplified project configuration: a project now has exactly one backend and an ensemble project must be used if you want to combine results from multiple backends
* 229 made the `fasttext` an optional dependency; the idea is that core Annif should work in a pure Python environment and any functionality that depends on native libraries should be an optional extra
* 37 new optional `voikko` analyzer for Finnish language, based on [libvoikko](https://voikko.puimula.org/)
* 228 migrated to new Ubuntu 16.04 Xenial environment when running unit tests in Travis CI; now running tests under Python 3.7 as well as 3.5 and 3.6

498 Update Dockerfile

Bug fixes:
484/495 Show error when training MLLM on empty corpus
489 Add Codecov Action to GH workflow for uploading reports
491 Raise NotSupportedException for attempt to train YAKE
497 Remove execute permissions of some files