refactor: move preprocess function to QueryDataset class and rename

[gcroci2]

Instead of populating a list with queries from the API, we use the former QueryDataset class (now QueryCollection class), which has a built-in method to append elements to the queries list (add method), and in which queries is defined as a property method of QueryCollection itself. A much cleaner way of doing the same thing.

Then I moved the former preprocess function (now process) to QueryCollection class as a method.