Switch to Connexion framework

[osma]

Use Connexion for REST API implementation, using the Swagger API spec to define the functionality

[tvirolai]

Note to self: here's how the Flask -> Connexion switch works without breaking existing functionality (ie. command-line interface implemented with Click).

First, create the Connexion app:
annif = connexion.App(__name__)

The Connexion app wraps a normal Flask app, which is here annif.app.

Then define the functions and map them to command-line commands like this:

@annif.app.cli.command('show-project')
@click.argument('projectid')
def show_project(projectid):
    asdflkjsdf

Lastly, add the Swagger API:

annif.add_api('api.yaml')

I'll try to get this implemented right away.