Added functionalities and comments in lib/csv.nit

Added methods : 

- 'get_header' -> allows to get the header from an existing CSV file and store it in the class 'header' property.

- 'read_line' -> allows to read a specific line from an existing CSV file.

- 'read_lines' -> allows to read all lines from an existing CSV file, excluding the header.

- 'read_all' enables to read all lines from an existing csv file including the header and returns
them in a String.

signed-off-by: Johan Kayser <johan.kayser@viacesi.fr>