Describe the bug
If we make a query using a datetime filter, the query searches for the given date between datetime or start_datetime, end_datetime or start and end_datetime. Below I'm including what the query looks like that the apply_datetime_filter function applies. This seems pointless to me - the user is asking about datetime, not about start_datetime and end_datetime fields. If the user is supposed to be able to search by these fields, they need to be added as separate search fields.

[Bool(filter=[Range(properties__datetime={'gte': '2016-05-23T10:41:10Z', 'lte': '2016-06-23T10:41:10Z'})]), Bool(filter=[Range(properties__start_datetime={'gte': '2016-05-23T10:41:10Z', 'lte': '2016-06-23T10:41:10Z'})]), Bool(filter=[Range(properties__end_datetime={'gte': '2016-05-23T10:41:10Z', 'lte': '2016-06-23T10:41:10Z'})]), Bool(filter=[Range(properties__start_datetime={'lte': '2016-05-23T10:41:10Z'}), Range(properties__end_datetime={'gte': '2016-06-23T10:41:10Z'})])]

To Reproduce
http://127.0.0.1:8080/search?collections=sentinel-2-l2a&datetime=2016-05-23T10:41:10.947Z/2016-06-23T10:41:10.947Z

Expected behavior
The API should return only dates between datetime and datetime, not use start_datetime and end_datetime