Add Canadian Overnight Repo Rate Average (CORRA)

[AND2797]

Add CORRA Index to QuantLib.

Closes #1685.

[boring-cyborg]

Thanks for opening this pull request! It might take a while before we look at it, so don't worry if there seems to be no feedback. We'll get to it.

[CLAassistant]

All committers have signed the CLA.

[coveralls]

coverage: 71.9% (-0.003%) from 71.903% when pulling d8e74fb on AND2797:master into c22f15a on lballabio:master.

[lballabio]

Thanks! The failing check is because the new files should also be added to the other build systems (the VC++ project and the autotools). Do you want me to do that, or do you want to give it a try?

[AND2797]

I see. No problem, I'll add it to those

[lballabio]

Thanks! I pushed a commit to your fork to reorder the lists so I didn't have to bother you again for a minor change. A suggestion for next time: it's better to open the PR from a branch, not master, so my pushes (if any) are less disruptive.

[AND2797]

Will keep it in mind!

[boring-cyborg]

Congratulations on your first merged pull request!