CSV loader not lazy

[garlontas]

Describe the bug
The CSV loader isn't lazy. It reads the content from the CSV file directly at call.

To Reproduce

from pystreamapi import Stream
from pystreamapi.loaders import csv

stream = Stream.of(csv("/path/to/data.csv")) # CSV is already read here. This is time and memory consuming!
stream.filter(...).map(...).for_each(...) # The terminal operation should actually trigger the CSV reader.

Expected behavior
The CSV file should be only read when the iterable (return of csv() function) is accessed.