Merge `get_xxx` and `get_all_xxx` methods

[hbredin]

I think it would simplify the client (from user perspective) to merge get_xxx and get_all_xxx methods (where xxx can be user, group, corpus, etc...)

Example for user methods

def get_user(self, user_id=None):
    route = 'user'
    if user_id is not None:
        route = route + '/' + user_id
    return self.get(route)

@juandelamontana @clbarras what do you think?

[johannpoignant]

Yes I think we could also simplified these routes :

get_user(id_user=<id_user>) -> return one user
get_user() -> return a list of users
idem for other resources

get_all_media_of_a_corpus -> get_media(id_corpus = <id_corpus>)
get_media -> get_media(id_media = <id_media>)
idem for layer and annotation

get_ACL_of_a_corpus -> get_corpus(id_corpus = <id_corpus>)
return ACL if the user is the OWNER of the ressource

update_user_ACL_of_a_corpus -> update_ACL_of_a_corpus(id_corpus = <id_corpus>, id_user = <id_user>)
idem for corpus/group, layer/user and layer/group