Set openalex to retrieve 50 DOIs per batch

[lwrubel]

This sets the batch size lower, since we discovered that we could have a batch of long DOIs that lead to the query string being longer than 4,096 characters. It could be possible to set it a little higher (75 was too high in current experience, but only ~50 chars too long).

This sets the batch size to 50, not passed in as a function parameter. Also opens up the possibility to skip pagination in the future, since the default OpenAlex per_page size is 50.