Fix funder filter #129

Previously filter funder used nested-terms function. This function can only be applied to nested filters (for example relation.type or award.number). Filter funder is not nested, so required a different function.

At the same time, this filter is used in function cayenne.data.funder/fetch-works, which requires filtering funders by multiple DOIs. For this reason, filter funder cannot used a simple equality function, as this function takes only one value.

To satisfy both use cases, this change introduces a new filter function multiple-equality, which can be run with a single or multiple filter values and handles non-nested filters.

Available at https://api_filter_funder.staging.crossref.org/works