Uses subclasses of BaseVertical rather than the Vertical enum to identify a vertical

[aborem]

Closes #61

So any time we were using Vertical.SubVerticalName we now use SubVerticalName, where SubVerticalName is a subclass of BaseVertical. Note that I do mean subclass and not instance of a subclass since we're using the classes themselves as identifiers of verticals. In the future, methods will be returning instances of those same subclasses.

I also adjusted the names of the vertical fetching methods to follow the structure: fetch_<vertical_name>_vertical for consistency and to avoid having to keep track of the plural names like before.

Misc

Anywhere where an argument arg to a function/method could have one or more items, like in the constructor of the exception classes, I make these into a *arg instead so we don't have to special case passing in different number of arguments.