[APIv1] Changes to Statistics

[jheubuch]

In my opinion it would be useful to change the data format of the api/v1/statistics endpoint.

In the "purpose" Array, the business types are delivered as a string (e.g. "private", "commute", ...). Instead it would be more useful to deliver those with the correlating Enum ID which is also used for e.g. checking into a connection (private = 0, commute = 2 etc).

Also the duration is delivered as a string. It could be delivered as a number.