Allow has_granules in search_datasets kwargs

[juliacollins]

This is somewhere between a bug and a feature request. I would like to use earthaccess.search_datasets() to retrieve collection metadata for collections that specifically do not have associated granules. The use case is the development of a workflow that generates the UMM-G metadata for the granules that will be archived, but are not yet available. Currently the DataCollections class sets has_granules to True for all cases, so no results are returned when I try to retrieve collection metadata for the dataset in question. The suggested change is to include has_granules in the kwargs accepted by search_datasets and DataCollections. I'll work on a pull request but wanted to get this idea in the hopper in case anyone sees a problem with it.

[itcarroll]

Closed in favor of #939.