Improve description for custom collector/provider

[raphaelrpl]

We should improve the documentation in how to create custom providers and attach it into collectors. It should contain the following entries:

• Add example for module initialization and class for this
• brief entrypoint management and how it is loaded by bdc-collectors

For example, consider an extension of current provider SciHub named MyProvider which uses different api location. You may have few ways to use this

• Change runtime parameter api_url:

from bdc_collectors.scihub import SciHub

catalog = SciHub(username="user", password="pass", api_url="https://my-url-provider-hub")
catalog.search(...)

# or
# from bdc_collectors.ext import CollectorExtension
# ext = CollectorExtension(app)  # flask_app
# catalog = ext.get_provider("SciHub")(username="user", password="pass", api_url="https://my-url-provider-hub")

• Custom Provider named MyProvider (in bdc-collectors bdc_collectors/my_provider/__init__.py or in other github repo):

from typing import Type

from ..base import BaseCollection
from ..scihub import SciHub

def init_provider():
    return dict(MyProvider=MyProviderAPI)


class MyProviderAPI(SciHub):
    def __init__(self, *args, **kwargs):
        kwargs.setdefault("api_url", "https://my-url-provider-hub")
        super(MyProviderAPI, self).__init__(*args, **kwargs)
    def get_collector(self, collection: str) -> Type[BaseCollection]:
        # ... logic for custom data collection
        return super().get_collector(collection) # optional

Register in setup.py the entrypoint for loading this provider:

entry_points={
        ...,
        'bdc_collectors.providers': [
            ...  # other providers
            'my_provider = bdc_collectors.my_provider'
        ]
}

and use it with:

from bdc_collectors.ext import CollectorExtension
ext = CollectorExtension(app)  # flask_app
catalog = ext.get_provider("MyProvider")(username="user", password="pass")