add get_one and get_many methods to paginated collection

[apollonin]

Adding more convenience to users when they want to get a predefined number of results from the paginated collection.

old way:

data_rows = dataset.data_rows()
data_row = next(data_rows)

new way:

data_row = dataset.data_rows().get_one()

old way:

data_rows = dataset.data_rows()
first_ten = []
for data_row in data_rows:
    first_ten.append(data_row)
    if len(first_ten) >= 10:
        break

new way:

data_rows = dataset.data_rows().get_many(10)

important note. These methods are idempotent. That means:

1. They can be called as many times as wanted and the result will be the same. E.x. get_one will always return first one result. It is not equivalent to calling next().
2. get_many will not call API in case it already has needed amount of data. This way to safe resources and time.