implement file paths

[jorgejesus]

Base provider class uses self.url to indicate where the provider is locate, this is problematic when we have for example spatialite that is a file.

Current we used file:// with asbolute paths. Need to create a specific file attriubte in the base class and support relative paths
e.g
self.file = "./data/"

[tomkralidis]

I've changed the config so that we use what's intuitive to the provider. self.url is now self.data of the dataset object (along with self.id_field and self.provider. So for GeoJSON/CSV, self.data is a filepath, whereas for Elasticsearch, self.data is the URL.