Adjustments to fetch API

[tijmenbaarda]

Adjust fetch API to support some requirements for the EDPOP VRE:

• Start fetching from an offset instead of from the beginning (this is needed because Reader objects won't be shared across API requests from the frontend, since every request might run in a different process)
• Adjust the maximum number of new results to fetch each time (this is needed to allow the user to control how many records they want to see)
• Merge the fetch() and fetch_next() methods

[jgonggrijp]

Quick comment: this seems like a solution for one of your open ends in CentreForDigitalHumanities/EDPOP#148. Next time you report a problem in one repository and work on a solution in another, please add a cross-reference at either end (like I'm doing here) so that it will be visible for future readers (like me) that you are already working on a solution.

[tijmenbaarda]

Quick comment: this seems like a solution for one of your open ends in UUDigitalHumanitieslab/EDPOP#148. Next time you report a problem in one repository and work on a solution in another, please add a cross-reference at either end (like I'm doing here) so that it will be visible for future readers (like me) that you are already working on a solution.

Sure, will do next time!!